# Asteroid User Guide

## Introduction

Asteroid is a modern, application-oriented, multi-paradigm programming language supporting first-class patterns. The language is heavily influenced by Python, Rust, ML, and Prolog. Furthermore, Asteroid is dynamically typed and makes pattern matching one of its core computational mechanisms. When we talk about pattern matching we mean both structural pattern matching as well as regular expression matching.

In this document we describe the major features of Asteroid and give plenty of examples. If you have used a programming language like Python or JavaScript before, then Asteroid should appear very familiar. However, there are some features which differ drastically from other programming languages due to the core pattern-matching programming paradigm with first-class patterns. Here are just two examples:

Example: All statements that look like assignments are actually pattern-match statements. For example if we state,

```let [x,2,y] = [1,2,3].
```

that means the list `[1,2,3]` is matched to the pattern `[x,2,y]` and `x` and `y` are bound to the values 1 and 3, respectively. By the way, there is nothing wrong with the following statement,

```let [1,2,3] = [1,2,3].
```

which is just another pattern match without any variable instantiations.

Example: Patterns in Asteroid are first-class citizens of the language. This is best demonstrated with a program. Here is a program that recursively computes the factorial of a positive integer and uses first-class patterns in order to ensure that the domain of the function is not violated,

```-- define first-class patterns
let pos_int = pattern (x:%integer) if x > 0.
let neg_int = pattern (x:%integer) if x < 0.

-- define our factorial function
function fact
with 0 do
return 1
with n:*pos_int do            -- use first pattern
return n * fact (n-1).
with n:*neg_int do            -- use second pattern
throw Error("undefined for "+n).
end
```

As you can see, the program first creates patterns and stores them in the variables `pos_int` and `neg_int` and it uses those patterns later in the code by dereferencing those variables with the `*` operator. First-class patterns have profound implications for software development in that pattern definition and usage points are now separate and patterns can be reused in different contexts.

These are just two examples where Asteroid differs drastically from other programming languages. This document is an overview of Asteroid and is intended to get you started quickly with programming in Asteroid.

## The Basics

As with most programming languages we are familiar with, Asteroid has variables (alpha-numeric symbols starting with an alpha character) and constants. Constants are available for all four primitive data types,

• `integer`, e.g. `1024`

• `real`, e.g. `1.75`

• `string`, e.g. `"Hello, World!"`

• `boolean`, e.g. `true`

Asteroid also supports the built-in data types:

• `list`

• `tuple`

These are structured data types in that they can contain entities that belong to other data types. Both of these data types have constructors which are sequences of comma separated values enclosed by square brackets for lists, e.g. `[1,2,3]`, and enclosed by parentheses for tuples, e.g. `(x,y)`. Lists are mutable structures in that we can add or delete elements. On the other hand, tuples are immutable objects; once created you cannot change them. Furthermore, for tuples we have the caveat that the 1-tuple is represented by a value followed by a comma to distinguish it from parenthesized expressions, e.g. `(3,)` the 1-tuple versus `(3)` the parenthesized expression. Here are some examples,

```let l = [1,2,3].  -- this is a list
let t = (1,2,3).  -- this is a tuple
```

As we said above, in order to distinguish it from a parenthesized value the single element in a 1-tuple has to be followed by a comma, like so,

```let one_tuple = (1,).  -- this is a 1-tuple
```

We can cast lists and tuples to strings for easy printing,

```load system io.
io @println ("this is my list: " + tostring [1,2,3]).
```

Here the `+` operator acts like a string concantenation operator with the list `[1,2,3]` promoted to a string.

Asteroid supports the `none` type. The `none` type has only one member: A constant named `none`. However, it turns out that the null-tuple, a tuple with no components indicated by `()`, also belongs to this type rather than the tuple type discussed earlier. But the `none` data type only has one constant, this implies that `()` and `none` mean the same thing and can be used interchangeably. That is, the following `let` statements will succeed,

```let none = ().
let () = none.
```

showing that `()` and `none` are equivalent and pattern-match each other.

We should mention here that because functions and patterns are both first-class citizens in Asteroid we also have the types `function` and `pattern`,

```-- define a function
function inc with x do
return x+1.
end

-- show that 'inc' is of type 'function'
assert (gettype(inc) == "function").
```

Here is a small program demonstrating the `pattern` data type,

```-- define a first-class pattern
let p = pattern (x:%integer) if x>0.

-- check the type of the value stored in p
assert (gettype(p) == "pattern").
```

Data types in Asteroid do not form type hierarchies as in C/C++ and Java, for example. Therefore, in mixed type arithmetic statements we have to explicitly convert data types as in,

```let x = 1.1 + toreal(1).
```

Asteroid shares this view of data types with prgogramming languages like SML and Rust.

By now you probably figured out that statements are terminated with a period and that comments start with a `--` symbol and continue till the end of the line. You probably also figured out that the `let` statement is Asteroid’s version of assignment even though the underlying mechanism is a bit different as we will see when we discuss pattern matching in more detail.

## Data Structures

### Lists

In Asteroid the `list` is a fundamental, built-in data structure. A trait it shares with programming languages such as Lisp, Python, ML, and Prolog. Below is a list reversal example program. Notice that lists are zero-indexed and elements of a list are accessed via the `@` operator,

```let a = [1,2,3].          -- construct list a
let b = [a@2, a@1, a@0].  -- reverse list a
assert (b == [3,2,1]).
```

We can achieve the same effect by giving a list of index values (a slice) to the `@` operator,

```let a = [1,2,3].     -- construct list a
let b = a@[2,1,0].   -- reverse list a using slice [2,1,0]
assert (b == [3,2,1]).
```

In Asteroid lists are considered objects with member functions that can manipulate list objects. We could rewrite the above example as,

```load system io.

let a = [1,2,3].
let b = a @reverse (). -- reverse list using member function 'reverse'
io @println b.
```

The `@` operator is Asteroid’s general access operator. It allows you to access either individual elements, slices, or member functions of a list. It also allows for access to members and functions of tuples and objects. Notice that in order to access the `println` function of the `io` module we also use the `@` operator. This is because in Asteroid, system modules are objects, so you must use `@` to access the functions of the module.

For a comprehensive treatment of available member functions for lists and tuples please see the reference guide. We look at objects later on in this guide.

Besides using the constructor for lists which consists of the square brackets enclosing comma separated elements we can use list comprehensions to construct lists. In Asteroid a list comprehension consist of a range specifier together with an optional step specifier allowing you to generate integer values within that range,

```load system io.

-- build a list of odd values
let a = [1 to 10 step 2].  -- list comprehension
io @println ("list: " + tostring a).

-- reverse the list using a slice computed as comprehension
let slice = [4 to 0 step -1]. -- list comprehension
let b = a@slice.
io @println ("reversed list: " + tostring b).
```

The output is,

```list: [1,3,5,7,9]
reversed list: [9,7,5,3,1]
```

Asteroid’s simple list comprehensions in conjunction with the `map` function for lists allows you to construct virtually any kind of list. For example, the following program constructs a list of alternating 1 and -1,

```load system io.

let a = [1 to 10] @map (lambda with x do math @mod (x,2))
@map (lambda with x do 1 if x==1 else -1).

io @println a.
```

where the output is,

```[1,-1,1,-1,1,-1,1,-1,1,-1]
```

### Tuples

The tuple is another fundamental, built-in data structure that can be found in Asteroid. Below is an example of a tuple declaration and access.

```let a = (1,2,3).    -- construct tuple a
let b = a@1.        -- access the second element in tuple a, tuples are 0-indexed
assert (b == 2).    -- assert that the value of the second element is correct
```

Lists and tuples may be nested,

```-- build a list of tuples
let b = [("a","b","c"),
("d","e","f"),
("g","h","i")].
-- Access an element in the nested structure.
assert (b@0@1 == "b").
```

Unlike lists, tuples are immutable. This means that their contents cannot be changed once they have been declared. The following program demonstrates this,

```load system io.

let b = ("a","b","c"). -- build a tuple

try
let b@1 = "z". -- attempt to modify an element in the tuple
catch Exception (kind,message) do
io @println (kind+": "+message).
end.
```

Which will print out the following message:

```SystemError: term '(a,b,c)' is not a mutable structure
```

Should we want to change the contents of an already declared tuple, we would need to abandon the original and create a new one with the updated contents. When to use tuples and when to use lists is really application dependent. Tuples tend to be preferred over lists when representing some sort of structure, like abstract syntax trees, where that structure is immutable meaning, for example, that the arity of a tree node cannot change.

### Structures and Objects

You can introduce custom data structures using the `structure` keyword. For example, the following statement introduces a structure of type `A` with data members `a` and `b`,

```structure A with
data a.
data b.
end
```

Structures differ from lists and tuples in the sense that the name of the structure acts like a type tag. So, when you define a new structure you are in fact introducing a new type into your program.

For each structure Asteroid creates a default constructor that instantiates an object from that structure. The default constructor copies the arguments given to it into the data member fields in the order that the arguments and data members appear in the program text. Also, the data fields of an object are accessed via their names rather than index values. Here is a simple example that illustrates all this,

```-- define a structure of type A
structure A with
data a.
data b.
end

let obj = A(1,2).     -- default constructor, a<-1, b<-2
assert (obj@a == 1).  -- access first data member
assert (obj@b == 2).  -- access second data member
```

The following is a more involved example,

```load system io.

structure Person with
data name.
data age.
data gender.
end

-- make a list of persons
let people = [
-- use default constructors to construct Person objects
Person("George", 32, "man"),
Person("Sophie", 46, "woman"),
Person("Oliver", 21, "man")
].

-- retrieve the second person on the list and use pattern
-- matching on Person objects to extract member values
let Person(name,age,gender) = people@1.

-- print out the member values
io @println (name + " is " + tostring age + " years old and is a " +  gender + ".").
```

The output is,

```Sophie is 46 years old and is a woman.
```

The `structure` statement introduces a data structure of type `Person` with the three data members `name`, `age`, and `gender`. We use this data structure to build a list of persons. One of the interesting things is that we can pattern match the generated data structure as in the second `let` statement in the program to extract information from a `Person` object.

In addition to the default constructor, structures in Asteroid also support user specified constructors and member functions. We’ll talk about those later when we talk about OO programming in Asteroid.

## The Let Statement

The `let` statement is a pattern matching statement of the form,

```let <pattern> = <value>.
```

where the pattern on the left side of the equal sign is matched against the value of the right side of the equal sign. When the pattern consist of just a single variable then the let statement can be viewed as Asteroid’s version of the assignment statement, e.g.,

```let x = 1.
```

However, statements like,

```let 1 = 1.
```

where we pattern match the pattern 1 on the left side to the value 1 on the right side are completely legal and highlight the fact that the let statement is not equivalent to an assignment statement.

Simple patterns are expressions that consist purely of constructors and variables. Constructors themselves consist of constants, list and tuple constructors, as well as user defined structures. The advantage of pattern matching is that it provides direct access to substructures of a particular value. This is often called “destructuring” of a value. Consider that we want to access the constituent values of the pair `(1,2)`. In a non-pattern-matching approach we would have to access each of these constituent values one-by-one,

```let p = (1,2).
let x = p@0.
let y = p@1.
assert (x==1 and y==2).
```

But in a pattern-matching approach we can write a `let` statement with a pattern that looks like a pair with the variables `x` and `y` where we expect our values to be,

```let p = (1,2).
let (x,y) = p.
assert (x==1 and y==2).
```

Matching the pattern against the value `(1,2)` stored in `p` first matches the pair structure against the pair value and then matches the variables to the appropriate substructures. Once the variables have been matched to value the `let` statement declares the variables in the current scope and they become available for computation.

The following is an example involving structures and objects,

```structure Person with
data name.
data age.
data profession.
end

let joe = Person("Joe", 32, "Cook").  -- construct an object
let Person(n,a,p) = joe.              -- pattern match object

assert (n=="Joe" and a==32 and p=="Cook").
```

We first construct an object `joe` with the first `let` statement and then use pattern matching to desctructure it with the second `let` statement binding its substructures to the variables `n`, `a`, and `p`.

Asteroid supports special patterns called type patterns that match any value of a given type. For instance, the `%integer` pattern matches any integer value. Here is a simple example,

```let %integer = 1.
```

This `let` statement succeeds because `1` is an integer value that can be pattern-matched against the type pattern `%integer`. Type pattern exist for all builtin data types, `%real` and `%list`. If you introduce a user defined type via a structure, then Asteroid will create a type pattern for all objects of that data type. Here is a simple example,

```structure Foo with
data a.
data b.
end

let %Foo = Foo(1,2).
```

Notice the type pattern for the user defined type `Foo` in the `let` statement.

Asteroid also supports conditional patterns. Here is an example where we to make sure that the variable `t` on the left matches a pair of integer values,

```let t if t is (%integer,%integer) = (1,2).
```

Of course, this `let` statement is going to be successful because the value on the right is indeed a pair of integers. This kind of conditional pattern appears so often in Asteroid code that Asteroid has a shorthand notation for this,

```let t:(%integer,%integer) = (1,2).
```

Again, here the `let` statement is only successful if `t` matches a pair of integers.

Shorthand conditional patterns often look like a variable declarations in other languages. In Asteroid, though, it is still just all about pattern matching. Consider,

```load system io.

let x:%real = math @pi.
io @println x.
```

The left side of the `let` statement is a conditional pattern that matches any real value, and if that match is successful then the value is bound to the variable `x`. The program will print the value `3.141592653589793`.

Beware of the fact that even though the `let` statement above looks like a declaration of a real variable it is not; it is a pattern match statement enforcing that the value assigned to `x` matches the pattern `%real`. Since this is a pattern match statement, this also means that standard type promotions such as promoting integers to reals during assignments in other programming languages do not apply here. For example, in Asteroid the following let statement fails,

```let x:%real = 1.
```

because `1` is an integer value and does not match the pattern `%real`.

## Flow of Control

Control structure implementation in Asteroid is along the lines of any of the modern programming languages such as Python, Swift, or Rust. For example, the `for` loop allows you to iterate over lists without having to explicitly define a loop index counter. In addition, the `if` statement defines what does or does not happen when certain conditions are met in a very familiar way. For a list of all control statements in Asteroid, please take a look at the reference guide.

As we said, in terms of flow of control statements there are really not a lot of surprises. This is because Asteroid supports loops and conditionals in a very similar way to many of the other modern programming languages. For example, here is a short program with a `for` loop that prints out the first six even positive integers,

```load system io.

for i in 0 to 10 step 2 do
io @println i.
end
```

The output is,

```0
2
4
6
8
10
```

Here is another example that iterates over lists,

```load system io.

let indexes = ["first","second","third"].
let birds = ["turkey","duck","chicken"].

for (ix,bird) in util @zip (indexes,birds) do
io @println ("the "+ix+" bird is a "+bird).
end
```

The output is,

```the first bird is a turkey
the second bird is a duck
the third bird is a chicken
```

In the loop we first create a list of pairs using the `zip` function, over which we then iterate while pattern matching on each of the pairs on the list with the pattern `(ix,bird)`.

The following is a short program that demonstrates an `if` statement,

```load system io.

let x = tointeger (io @input "Please enter an integer: ").

if x < 0 do
let x = 0.
io @println "Negative, changed to zero".
elif x == 0 do
io @println "Zero".
elif x == 1 do
io @println "One".
else do
io @println "Something else".
end
```

Even though Asteroid’s flow of control statements look so familiar, they support pattern matching to a degree not found in other programming languages and which we will take a look at below.

## Functions

Functions in Asteroid resemble function definitions in functional programming languages such as Haskell and ML. Here functions definitions have a single formal argument and function calls are expressed via juxtaposition of the function name and the single actual argument. Here is a simple example,

```function double with i do -- pattern match the actual arg with i
return 2*i.
end

let d = double 2.  -- function call via juxtaposition, no parentheses necessary
assert (d == 4).
```

In the `with` expression we pattern match the actual argument that is being passed in against the variable `i`. Also note that the function call is expressed via juxtaposition, no parentheses necessary.

If we wanted to pass more than a single value to a function we have to create a tuple and then pass that tuple to the function like in this example,

```function reduce with (a,b) do -- pattern match the actual argument
return a*b.
end

let r = reduce (2,4).  -- function call via juxtaposition
assert (r == 8).
```

Even though the function call looks like a traditional function call like in Python it is not. The underlying mechanism is quite different: on the call site we construct a tuple that holds all our values which is then passed to the function as the only parameter. Within the function that tuple is pattern matched and whatever variables are instantiated during this pattern match can be used within the function body.

In Asteroid functions are multi-dispatch, that is, a single function can have multiple bodies each attached to a different pattern matching the actual argument. The following is the quick sort implemented in Asteroid where each `with` clause introduces a new pattern with its corresponding function body,

```load system io.

function qsort
with [] do -- empty list pattern
return [].
with [a] do -- single element list pattern
return [a].
with [pivot|rest] do -- separating the list into pivot and rest of list
let less=[].
let more=[].

for e in rest do
if e < pivot do
less @append e.
else
more @append e.
end
end

return qsort less + [pivot] + qsort more.
end

-- print the sorted list
io @println (qsort [3,2,1,0])
```

The output is as expected,

```[0,1,2,3]
```

Notice that we use the multi-dispatch mechanism to deal with the base cases in the first two `with` clauses. In the third `with` clause we use the pattern `[pivot|rest]` to match the input list. Here the variable `pivot` matches the first element of the list, and the variable `rest` matches the remaining list. This remaining list is the original list with its first element removed. The function body then implements the pretty much standard definition of the quick sort. Just keep in mind that function calls are expressed via juxtaposition of function name and actual argument; no parentheses necessary.

As you have seen in a couple of occasions already in the document, Asteroid also supports anonymous or `lambda` functions. Lambda functions behave just like regular functions except that you declare them on-the-fly and they are declared without a name. Here is an example using a `lambda` function,

```load system io.

io @println ((lambda with n do n+1) 1).
```

The output is `2`. Here, the `lambda` function is a function that takes a value and increments it by one. We then apply the value `1` to the function and the print function prints out the value `2`.

## Pattern Matching

Pattern matching lies at the heart of Asteroid. We saw some of Asteroid’s pattern matching ability when we discussed the `let` statement. Here is a more general discussion of pattern matching.

### Pattern Matching in Expressions: The Is Predicate

We can also have pattern matching in expressions using the `is` predicate. The left operand of the `is` predicate is a term and the right operand is a pattern. If the pattern match succeeds the predicate will return `true` otherwise it will return `false`. Consider the following example,

```load system io.

let p = (1,2).

if p is (x,y,z) do
io @println ("it's a triple with: "+ tostring x +","+ tostring y +","+ tostring z)
elif p is (x,y) do
io @println ("it's a pair with: "+ tostring x +","+ tostring y).
else do
io @println "it's something else".
end
```

Here we use patterns to determine if `p` is a triple, a pair, or something else. Pattern matching is embedded in the expressions of the `if` statement using the `is` predicate. The output of this program is,

```it's a pair with: 1,2
```

Pattern matching with the `is` predicate can happen anywhere expressions can be used. That means we can use the predicate also on the right side of `let` statements,

```let true = (1,2) is (1,2).
```

This is kind of strange looking but it succeeds. Obviously this pattern match will succeed because the term and the pattern look identical. The return value of the `is` predicate is then pattern matched against the `true` pattern on the left of the `=` symbol.

We can also employ pattern matching in loops. In the following program we use the `is` predicate to test whether a list is empty or not while looping,

```load system io.

let list = [1,2,3].

repeat do
let list = tail.
until list is []. -- pattern match with is predicate
```

The output is,

```1
2
3
```

The example employs pattern matching using the head-tail operator in order to iterate over the list elements and print them. The termination condition of the loop is computed with the `is` predicate.

### Pattern Matching in Function Arguments

As we have seen earlier, Asteroid supports pattern matching on function arguments in the style of ML and many other functional programming languages. Here is an example that uses pattern matching on function arguments using binary tree data structures,

```structure Node with -- internal tree node with a value
data value.
data left_child.
data right_child.
end

structure Leaf with -- leaf node with a value
data value.
end

-- traverse a tree and collect all the values in the tree in a list
function traverse
with Leaf(v) do
return [v].
with Node(v,l,r) do
return [v] + traverse l + traverse r.
end

let tree = Node(1,Leaf(2),Leaf(3)).
assert (traverse(tree) == [1,2,3]).
```

The structures `Node` and `Leaf` allow us to construct binary trees with embedded values. The `traverse` function traverses such trees and collects the values embedded in a tree on a list and returns that list. Notice the pattern matching on the tree node constructs in the `with` clauses of the `traverse` funtion.

### Conditional Pattern Matching

Asteroid allows the user to attach conditions to patterns that need to hold in order for the pattern match to succeed. This is particularly useful for restricting input values to function bodies. Consider the following definition of the `factorial` function where we use conditional pattern matching to control the kind of values that are being passed to a particular function body,

```load system io.

function factorial
with 0 do
return 1
with n if (n is %integer) and (n > 0) do
return n * factorial (n-1).
with n if (n is %integer) and (n < 0) do
throw Error("factorial is not defined for "+n).
end

io @println ("The factorial of 3 is: " + tostring (factorial 3)).
```

Here we see that first, we make sure that we are being passed integers and second, that the integers are positive using the appropriate conditions on the input values. If we are being passed a negative integer, then we throw an error.

The above factorial program can be simplified by rewriting the first condition on `n` in the conditional patterns as a named pattern. We can also take advantage of the fact that the last expression evaluated in a function body provides an implicit return value. This gives us,

```load system io.

function factorial
with 0 do
1
with (n:%integer) if n > 0 do
n * factorial (n-1).
with (n:%integer) if n < 0 do
throw Error("factorial is not defined for "+n).
end

io @println ("The factorial of 3 is: " + tostring (factorial 3)).
```

The parentheses as they appear in the conditional pattern expressions are necessary.

### Pattern Matching in For Loops

We have seen pattern matching in `for` loops earlier. Here we show another example. This combines structural matching with regular expression matching in `for` loops that selects certain items from a list. Suppose we want to print out the names of persons that contain a lower case ‘p’,

```load system io.

structure Person with
data name.
data age.
end

-- define a list of persons
let people = [
Person("George", 32),
Person("Sophie", 46),
Person("Oliver", 21)
].

-- print names that contain 'p'
for Person(name if name is ".*p.*",age) in people do
io @println name.
end
```

Here we pattern match the `Person` object in the `for` loop and then use a regular expression to see if the name of that person matches our requirement that it contains a lower case ‘p’. The output is `Sophie`.

### Pattern Matching in Try-Catch Statements

Exception handling in Asteroid is very similar to exception handling in many of the other modern programming languages available today. The example below shows an Asteroid program that throws one of two exceptions depending on the randomly generated value `i`,

```load system io.

data val.
end

structure Tail with
data val.
end

try
let i = random @random ().
if i >= 0.5 do
else do
throw Tail i.
end
io @println ("you win with "+tostring (v,stringformat (4,2))).
catch Tail v do
io @println ("you loose with "+tostring (v,stringformat (4,2))).
end
```

The `Head` and `Tail` exceptions are handled by their corresponding `catch` statements, respectively. In both cases the exception object is unpacked using pattern matching and the unpacked value is used in the appropriate message printed to the screen.

It is worth noting that even though Asteroid has builtin exception objects such as `Error`, you can construct any kind of object and throw it as part of an exception.

## Structures, Object-Oriented Programming, and Pattern Matching

We saw structures such as,

```structure Person with
data name.
data age.
end
```

earlier. It is Asteroid’s way to create custom data structures. These structures introduce a new type name into a program. For instance, in the case above, the `structure` statement introduces the type name `Person`. Given a structure definition, we can create objects from that structure. For example,

```let scarlett = Person("Scarlett",28).
```

The right side of the `let` statement invokes the default constructor for the structure in order to create an object stored in the variable `scarlett`. We can access members of the object,

```load system io.

structure Person with
data name.
data age.
end

let scarlett = Person("Scarlett",28).
-- access the name field of the structure instance
io @println (scarlett@name).
```

Asteroid allows you to attach functions to structures. In member functions the object identity is available through the `this` keyword. For example, we can extend our `Person` structure with the `hello` function that uses the `name` field of the object,

```load system io.

structure Person with
data name.
data age.
function hello with none do
io @println ("Hello, my name is "+this@name).
end
end

let scarlett = Person("Scarlett",28).
-- call the member function
scarlett @hello ().
```

This program will print out,

```Hello, my name is Scarlett
```

The expression `this@name` accesses the `name` field of the object the function `hello` was called on. Even though our structures are starting to look a bit more like object definitions, pattern matching continues to work in the same way from when we discussed structures. The only thing you need to keep in mind is that you cannot pattern match on a function member field. From a pattern matching perspective, a structure consists only of data fields. So even if we declare a structure like this,

```load system io.

structure Person with
data name.
-- the function is defined in the middle of the data fields
function hello with none do
io @println ("Hello, my name is "+this@name).
end
data age.
end

-- pattern matching ignores function definitions
let Person(name,age) = Person("Scarlett",28).
io @println (name+" is "+ tostring age+" years old").
```

where the function `hello` is defined in the middle of the data fields, pattern matching simply ignores the function definition and pattern matches only on the data fields. The output of the program is,

```Scarlett is 28 years old
```

Here is a slightly more involved example loosely based on the dog example from the Python documentation. The idea of the dog example is to have a structure that describes dogs by their names and the tricks that they can perform. Rather than using the default constructor, we define a constructor for our instances with the `__init__` function that performs some basic type checking on its arguments using type patterns and then initializes the data members of the object. Here is the program listing for the example in Asteroid,

```load system io.

structure Dog with
data name.
data tricks.
function __init__ with (name:%string, tricks:%list) do -- constructor
let this@name = name.
let this@tricks = tricks.
end
end

let buddy = Dog("Buddy",["sit stay","roll over"]).
let bella = Dog("Bella",["roll over","fetch"]).

let dogs = [fido,buddy,bella].

-- print out all the dogs that know how to fetch
for (Dog(name,tricks) if tostring tricks is ".*fetch.*") in dogs do
io @println (name+" knows how to fetch").
end
```

After declaring the structure, we instantiate the dogs with their respective trick repertoires and we then put them on a list. The last couple of lines of the program consist of a `for` loop over the list of our dogs. The `for` loop is interesting because here we use structural, conditional, and regular expression pattern matching in order to only select the dogs that know how to do `fetch` from the list of dogs. The pattern is,

```Dog(name,tricks) if tostring tricks is ".*fetch.*"
```

The structural part of the pattern is `Dog(name,tricks)` which simply matches any dog instance on the list. However, that match is only successful if the conditional part of the pattern holds,

```if tostring tricks is ".*fetch.*"
```

This condition only succeeds if the `tricks` list viewed as a string matches the regular expression `".*fetch.*"`. That is, if the list contains the word `fetch`. The output is,

```Fido knows how to fetch
Bella knows how to fetch
```

## Patterns as First-Class Citizens

A programming language feature that is promoted to first-class status does not change the power of a programming language in terms of computability but it does increase its expressiveness. Think functions as first-class citizens of a programming language. First-class functions give us `lambda` functions and `map`, both powerful programming tools.

The same is true when we promote patterns to first-class citizen status in a language. It doesn’t change what we can and cannot compute with the language. But it does change how we can express what we want to compute. That is, it changes the expressiveness of a programming language.

In Asteroid first-class patterns are introduced with the keyword `pattern` and patterns themselves are values that we can store in variables and then reference when we want to use them. Like so,

```let p = pattern (x,y).  -- define a first-class pattern
let *p = (1,2).         -- use the first-class pattern
```

The left side of the second `let` statement dereferences the pattern stored in variable `p` and uses the pattern to match against the value `(1,2)`.

Here we look at three examples of how first-class patterns can add to a developer’s programming toolbox.

### Pattern Factoring

Patterns can become very complicated especially when conditional pattern matching is involved. First-class patterns allow us to control the complexity of patterns by breaking patterns up into smaller subpatterns that are more easily managed. Consider the following function that takes a pair of values. The twist is that the first component of the pair is restricted to primitive data types of Asteroid,

```function foo with (x if (x is %boolean) or (x is %integer) or (x is %string),y) do
io @println (x,y).
end
```

That complicated pattern for the first component of the input pair completely obliterates the overall structure of the parameter pattern and makes the function definition difficult to read.

We can express the same function with a first-class pattern,

```let tp = pattern q if (q is %boolean) or
(q is %integer) or
(q is %string).

function foo with (x:*tp,y) do
io @println (x,y).
end
```

It is clear now that the main input structure to the function is a pair and the conditional type restriction pattern has been relegated to a subpattern stored in the variable `tp`.

### Pattern Reuse

In most applications of patterns in programming languages specific patterns appear in many spots in a program. If patterns are not first-class citizens the developer will have to retype the same patterns over and over again in the various different spots where the patterns occurs. Consider the following program snippet,

```function fact
with 0 do
return 1
with (n:%integer) if n > 0 do
return n * fact (n-1).
with (n:%integer) if n < 0 do
throw Error("fact undefined for negative values").
end

function sign
with 0 do
return 1
with (n:%integer) if n > 0 do
return 1.
with (n:%integer) if n < 0 do
return -1.
end
```

In order to write these two functions we had to repeat the almost identical pattern four times. First-class patterns allow us to write the same two functions in a much more elegant way,

```let pos_int = pattern (x:%integer) if x > 0.
let neg_int = pattern (x:%integer) if x < 0.

function fact
with 0 do
return 1
with n:*pos_int do
return n * fact (n-1).
with *neg_int do
throw Error("fact undefined for negative values").
end

function sign
with 0 do
return 1
with *pos_int do
return 1.
with *neg_int do
return -1.
end
```

The relevant patterns are now stored in the variables `pos_int` and `neg_int` which are then used in the function definitions.

### Patterns as Constraints

Sometimes we want to use patterns as constraints on other patterns. Consider the following example,

```let x: v if (v is %integer) and (v > 0) = some_value.
```

Here we want to use the pattern `v if (v is %integer) and (v > 0)` purely as a constraint on the pattern `x` in the sense that we want a match on `x` only to succeed if the variable `some_value` holds a positive integer. The problem is that this pattern introduces a spurious binding of the variable `v` into the current environment which might be undesirable due to variable name clashes. We can rewrite the above statement using the pattern scope operator `%[...]%` as follows,

```let x: %[v if (v is %integer) and (v > 0) ]% = some_value.
```

By placing the pattern `v if (v is %integer) and (v > 0)` within the `%[...]%` scope operator the pattern still functions as before but does not bind the variable `v` into the current environment.

The most common use of patterns as constraints is the prevention of non-linear patterns in functions. Consider the following program,

```load system io.

let pos_int = pattern %[v if (v is %integer) and (v > 0)]%.

return a+b.
end

```

Without the `%[...]%` scope operator the pattern `v if (v is %integer) and (v > 0)` the argument list pattern for the function `(a:*pos_int,b:*pos_int)` would instantiate two instances of the variable `v` leading to a non-linear pattern which is not supported by Asteroid. With the `%[...]%` scope operator in place we prevent the pattern `v if (v is %integer) and (v > 0)` from instantiating the variable `v` thus preventing a non-linearity to occur in the argument list pattern.

Sometimes we need to use patterns as constraints instead of straightforward patterns in order to avoid non-linearities but we also want controlled access to the variables these constraint patterns declare. We achieve this by using the `bind` keyword at the pattern-match site. Consider the following program,

```-- declare a pattern that matches scalar values
let scalar = pattern %[p if (p is %integer) or (p is %real)]%.

-- declare a pattern that matches pairs of scalars
let pair = pattern %[(x:*scalar,y:*scalar)]%.

-- compute the dot product of two pairs of scalars
function dot2d
with (*pair bind [x as a1, y as a2], *pair bind [x as b1, y as b2]) do
a1*b1 + a2*b2
end

assert(dot2d((1,0),(0,1)) == 0).
```

In the function definition of `dot2d` we see that the `pair` pattern is used twice to make sure that the function is called with a pair of pairs as its argument. However, to compute the dot product of those two pairs we need access to the values each pair matched. We use the `bind` keyword together with an appropriate binding term list to extract the matched values. For the first pair we map `x` and `y` to `a1` and `a2` and for the second pair we map `x` and `y` to `b1` and `b2`, respectively.

As a quick aside, the `as` construction in the binding term list is only necessary when trying to resolve non-linearities otherwise the binding term list can just consist of the variable names appearing in the pattern that you want to bind into the current scope.

### Notes on First-Class Patterns

It is important to remember that first-class patterns act like macros or dynamically scoped functions when they are used. That is, anything that is referenced from within the pattern needs to be defined in the environment where the pattern is actually used! That is especially true for patterns defined in different modules.

Consider the following code,

```function foo with none do
load system math.  -- math only available in the function local scope
return pattern %[(x:%integer) if math@mod(x,2) == 0]%.
end

let even_pattern = foo().
let n:*even_pattern = 2.  -- Error: module 'math' not defined
```

This code will fail with the error,

```error: 'math' is not defined
```

Here, we construct a first-class pattern in the local scope of the function `foo` and return it to the caller. Since patterns are only evaluated at their usage points the `math` module in the function has no effect. Furthermore, when finally applying the pattern in the last `let` statement the code will fail since the `math` module is not defined in that scope.

To remedy the situation we have to move the loading of the `math` module into the scope where the pattern is used,

```function foo with none do
return pattern %[(x:%integer) if math@mod(x,2) == 0]%.
end

load system math.  -- now 'math' is available in the scope of the pattern usage
let even_pattern = foo().
let n:*even_pattern = 2.
```

## Basic Asteroid I/O

I/O functions are defined as member functions of the `io` module. The `println` function prints its argument in a readable form to the terminal. Recall that the `+` operator also implements string concatenation. This allows us to construct nicely formatted output strings,

```load system io.

let a = 1.
let b = 2.
io @println ("a + b = " + tostring (a + b)).
```

The output is

```a + b = 3
```

We can use the builtin `tostring` function to provide some additional formatting. The idea is that the `tostring` function takes a value to be turned into a string together with an optional `stringformat` formatting specifier object,

```tostring(value[,stringformat(width spec[,precision spec])])
```

Here the structures appearing in square brackets are optional. The width specifier tells the `tostring` function how many characters to reserve for the string conversion of the value. If the value requires more characters than given in the width specifier then the width specifier is ignored. If the width specifier is larger than than the number of characters required for the value then the value will be right justified. For real values there is an optional precision specifier.

Here is a program that exercises some of the string formatting options,

```load system io.

-- if the width specifier is larger than the length of the value
-- then the value will be right justified
let b = tostring(true,stringformat(10)).
io @println b.

let i = tostring(5,stringformat(5)).
io @println i.

-- we can format a string by applying tostring to the string
let s = tostring("hello there!",stringformat(30)).
io @println s.

-- for floating point values: first value is width, second value precision.
-- if precision is missing then value is left justified and zero padded on right.
let r = tostring(math@pi,stringformat(6,3)).
io @println r.
```

The output of the program is,

```     true
5
hello there!
3.142
```

Notice the right justification of the various values within the given string length.

The `io` module also defines a function `print` which behaves just like `println` except that it does not terminate print with a newline.

Another useful function defined in the `io` module is the `input` function that, given an optional prompt string, will prompt the user at the terminal and return the input value as a string. Here is a small example,

```load system io.

let name = io @input("What is your name? ").
io @println ("Hello " + name + "!").
```

The output is,

```What is your name? Leo
Hello Leo!
```

We can use the builtin type casting functions such as `tointeger` or `toreal` to convert the string returned from `input` into a numeric value,

```load system io.

let i if i > 0  = tointeger(io @input("Please enter a positive integer value: ")).

for k in 1 to i do
io @println k.
end
```

The output is,

```Please enter a positive integer value: 3
1
2
3
```

## The Module System

A module in Asteroid is a file with a set of valid Asteroid statements. You can load this file into other Asteroid code with the statement:

```load "example_path/example_filename".
```

or:

```load example_modulename.
```

The search strategy for a module to be loaded is as follows,

1. raw module name - could be an absolute path

2. search in current directory

3. search in directory where Asteroid is installed

4. search in subdirectory where Asteroid was started

Modules defined by the Asteroid system should be loaded with the keyword `system` in order to avoid any clashes with locally defined modules. If the `system` keyword is used then Asteroid only searches in its system folders rather than in user directories.

Say that you wanted to load the `math` module so you could execute a certain trigonometric function. The following Asteroid program loads the `math` module as well as the `io` module. Only after loading them would you be able to complete the sine function below,

```load system io.

let x = math @sin( math@pi / 2.0 ).
io @println("The sine of pi / 2 is " + tostring x + ".").
```

Both the function `sin` and the constant value `pi` are defined in the `math` module. In addition, the `io` module is where all input/output functions in Asteroid (such as `println`) come from. If you want the complete list of modules, make sure to check out the reference guide.

## More on Exceptions

This section will give further information on how to work with exceptions, or unexpected conditions that break the regular flow of execution. Exceptions generated by Asteroid are `Exception` objects with the following structure,

```structure Exception with
data kind.
data value.
end
```

The `kind` field will be populated by Asteroid with one of the following strings,

• `PatternMatchFailed` - this exception will be thrown if the user attempted an explicit pattern match which failed, e.g. a let statement whose left side pattern does not match the term on the right side.

• `NonLinearPatternError` - this exception occurs when a pattern has more than one variable with the same name, e.g. `let (x,x) = (1,2).`

• `RedundantPatternFound` - this exception is thrown if one pattern makes another superfluous, e.g. in a multi-dispatch function definition.

• `ArithmeticError` - e.g. division by zero

• `FileNotFound` - an attempt of opening a file failed.

• `SystemError` - a general exception.

In addition to the `kind` field, the `value` field holds a string with some further details on the exception. Specific exceptions can be caught by pattern matching on the `kind` field of the `Exception` object. For example,

```load system io.

try
let x = 1/0.
catch Exception("ArithmeticError", s) do
io @println s.
end
```

The output is,

```integer division or modulo by zero
```

Asteroid also provides a predefined `Error` object for user level exceptions,

```load system io.

try
throw Error("something worth throwing").
catch Error(s) do
io @println s.
end
```

Of course the user can also use the `Exception` object for their own exceptions by defining a `kind` that does not interfere with the predefined `kind` strings above,

```load system io.

try
throw Exception("MyException","something worth throwing").
catch Exception("MyException",s) do
io @println s.
end
```

The output here is,

```something worth throwing
```

In addition to the Asteroid defined exceptions, the user is allowed to construct user level exceptions with any kind of object including tuples and lists. Here is an example that constructs a tuple as an exception object,

```load system io.

try
throw ("funny exception", 42).
catch ("funny exception", v) do
io @println v.
end
```

The output of this program is `42`.

Now, if you don’t care what kind of exception you catch, you need to use a `wildcard` or a variable because exception handlers are activated via pattern matching on the exception object itself. Here is an example using a `wildcard`,

```load system io.

try
let (x,y) = (1,2,3).
catch _ do
io @println "something happened".
end
```

Here is an example using a variable,

```load system io.

try
let (x,y) = (1,2,3).
catch e do
io @println ("something happened: "+ tostring e).
end
```

In this last example we simply convert the caught exception object into a string and print it,

```something happened: Exception(PatternMatchFailed,pattern match failed: term and pattern
lists/tuples are not the same length)
```

## More on Multi-Dispatch

With the `qsort` function above we saw functional programming style dispatch where the `with` clauses represent a case analysis over a single type, namely the input type to the function. However, Asteroid has a much broader view of multi-dispatch where the `with` clauses can represent a case analysis over different types. In order to demonstrate this type of multi-dispatch, we show the example program from the multi-dispatch Wikipedia page written in Asteroid,

```load system io.

let pos_num = pattern %[x if isscalar(x) and x > 0]%.

structure Asteroid with
data size.
function __init_
with v:*pos_num do
let this@size = v.
end
end

structure Spaceship with
data size.
function __init_
with v:*pos_num do
let this@size = v.
end
end

-- we use first-class pattern SpaceObject to
-- express that both asteroids and space ships are space objects.
let SpaceObject = pattern %[x if (x is %Asteroid) or (x is %Spaceship)]%.

-- multi-dispatch function
function collide_with
with (a:%Asteroid, b:%Spaceship) do
return "a/s".
with (a:%Spaceship, b:%Asteroid) do
return "s/a".
with (a:%Spaceship, b:%Spaceship) do
return "s/s".
with (a:%Asteroid, b:%Asteroid) do
return "a/a".
end

-- here we use the first-class pattern SpaceObject as a
-- constraint on the function parameters.
function collide with (x:*SpaceObject, y:*SpaceObject) do
return "Big boom!" if (x@size > 100 and y@size > 100) else collide_with(x, y).
end

io @println (collide(Asteroid(101), Spaceship(300))).
io @println (collide(Asteroid(10), Spaceship(10))).
io @println (collide(Spaceship(101), Spaceship(10))).
```

Each `with` clause in the function `collide_with` introduces a new pattern with its corresponding function body. Each pattern represents a different data type. In this case different kinds of pairs. The function bodies in this case are simple `return` statements but they could be arbitrary computations. The output of the program is,

```Big boom!
a/s
s/s
```