Carp/docs/LanguageGuide.md

## The Language

### Introduction

Carp borrows its looks from Clojure but the runtime semantics are much closer to those of ML or Rust.
Types are inferred but can be annotated for readability using the `the` keyword (see below).

Memory management is handled by static analysis, a value is owned by the function where it was created.
When a value is returned or passed to another function the initial function will give up ownership of it
and any subsequent use will lead to a compiler error. To temporarily lend a value to another function
(for example to print it) a reference must be created, using the `ref` special form (or the `&` reader macro).

To learn more about the details of memory management, check out [Memory.md](https://github.com/carp-lang/Carp/blob/master/docs/Memory.md)

### Comments
```clojure
;; Comments begin with a semicolon and continue until the end of the line.
```

### Data Literals
```clojure
100     ;; Int
1500l   ;; Long
3.14f   ;; Float
10.0    ;; Double
1b      ;; Byte
true    ;; Bool
"hello" ;; &String
#"hello" ;; &Pattern
\e      ;; Char
[1 2 3] ;; (Array Int)
{1 1.0 2 2.0} ;; (Map Int Double)
```

### Type Literals
```clojure
t ;; Type variables begin with a lowercase letter
(f t) ;; Type constructor variables; matches `(Maybe Int)` but not `Int`
Int
Long
Float
Double
Byte
Bool
String
Pattern
Char
(Array t)
(Map <key-type> <value-type>)
(Fn [<arg-type1> <arg-type2> ...] <return-type>) ;; Function type
```

### Dynamic-only Data Literals
Right now the following data types are only available for manipulation in non-compiled code.

```clojure
(1 2 3) ; list
foo ; symbol
```

### Defining things
```clojure
(defn function-name [<arg1> <arg2> ...] <body>) ;; Define a function (will be compiled, can't be called at the REPL)
(definterface interface-name (Fn [<t1> <t2>] <return>)) ;; Define a generic function that can have multiple implementations
(def variable-name value) ;; Define a global variable (only handles primitive constants for the moment)
(defmacro <name> [<arg1> <arg2> ...] <macro-body>) ;; Define a macro, its argument will not be evaluated when called
(defdynamic <name> <value>) ;; A variable that can only be used at the REPL or during compilation
(defndynamic <name> [<arg1> <arg2> ...] <function-body>) ;; A function that can only be used at the REPL or during compilation
(defmodule <name> <definition1> <definition2> ...) ;; The main way to organize your program into smaller parts
```

#### Interfaces

Interfaces specify a generic function signature that multiple concrete
functions may implement. You can define an interface using
`definterface`, passing a name and type signature of a function:

```clojure
(definterface speak (Fn [a] String))
```

You can declare a function as an implementation of an interface using
`implements`. For example, the following snippet declares `Dog.bark`
and `Cat.meow` as an implementation of `speak`:

```clojure
(definterface speak (Fn [a] String))

(defmodule Dog
  (defn bark [aggressive?]
    (if aggressive? @"WOOF!" @"woof!"))
  (implements speak Dog.bark))

(defmodule Cat
  (defn meow [times] (String.repeat times "meow!"))
  (implements speak Cat.meow))
```

Only functions that satisfy an interface's singature can implement
it. For exmaple, the following function isn't a valid implementation
of `speak` because it has the wrong number of arguments and its return
type does not match the return type of `speak`:

```clojure
(defmodule Number
  ;; who knew numbers could talk?
  (defn holler [] "WOO!")
  (implements speak Number.holler))
=> [INTERFACE ERROR] Number.holler : (Fn [] (Ref String a)) doesn't match the interface signature (Fn [a] String)
```

When you call an interface by name, Carp uses the current context and
the type signature of each implementation to call an implementation
that type checks:

```clojure
(speak 2) ;; Int -> String, Cat.meow
=> "meow!meow!"
(speak false) ;; Bool -> String, Dog.bark
=> "woof!"
```

If more than one interface implementation satisfies Carp's type
checker in a given context, Carp will complain about the ambiguity:

```clojure
(defmodule Pikachu
  (defn pika [times] (String.repeat times "pika!"))
  (implements speak Pikachu.pika))

(speak 2) ;; Int -> String, Cat.meow OR Pikachu.pika
=> There are several exact matches for the interface `speak` of type `(Fn [Int] String)` at line 1, column 2 in 'REPL'
Possibilities:
    Cat.meow : (Fn [Int] String)
    Pikachu.pika : (Fn [Int] String) at REPL:1:1.
```

In such cases, you'll have to help the Carp compiler disambiguate the
call by calling the implementing function you need
directly. It usually isn't useful to provide multiple
implementations that have the same function signature.

### Conditional statements with `cond`
The `cond` statement executes a block of code if a specified condition is true. If the condition is false, another block of code can be executed.

```clojure
(doc cond "this is the documentation for cond")
```

Usage:

```clojure
(cond
          (<condition_1>) (<code_1>) ;; code_1 gets executed if condition_1 is true
          (<condition_2>) (<code_2>) ;; code_2 gets executed if condition_2 is true
          (<code_3>) ;; code_3 gets executed if condition_1 and condition_2 are false
```

Here's an example about printing a statement depending on whether it is < or > 10:

```clojure
(cond
  (< 10 1) (println "Don't print!")
  (> 10 1) (println msg)
  (println "Don't print!"))
```

### Special Forms
The following forms can be used in Carp source code and will be compiled to C after type checking
and other static analysis. The first three of them are also available in dynamic functions.

```clojure
(fn [<arg1> <arg2> ...] <body>) ;; Create a lambda function (a.k.a. closure)
(let [<var1> <expr1> <var2> <expr2> ...] <body>) ;; Create local bindings
(do <expr1> <expr2> ... <return-expression>) ;; Perform side-effecting functions, then return a value
(if <expression> <true-branch> <false-branch>) ;; Branching
(while <expression> <body>) ;; Loop until expression is false
(use <module>) ;; Brings all symbols inside <module> into the scope
(with <module> <expr1> <expr2> ...) ;; Locally scoped `use` statement where all expressions after it will look up symbols in the <module>
(match <expression> <case1> <expr1> <case2> <expr2> ...) ;; Pattern matches <expression> against a set of sumtype constructors
(ref <expression>) ;; Borrow an owned value
(address <expression>) ;; Takes the memory address of a value, returns a C-style pointer
(set! <variable> <expression>) ;; Mutate a variable
(the <type> <expression>) ;; Explicitly declare the type of an expression
```

Here's an example of how to use the `the` form to make an identity function that only accepts Integers:

```clojure
(defn f [x]
  (the Int x))
```

### Reader Macros
```clojure
&x ;; same as (ref x)
@x ;; same as (copy x)
```

### Named Holes
When using a statically typed language like Carp it can sometimes be hard to know what value should
be used at a specific point in your program. In such cases the concept of 'holes' can be useful. Just
add a hole in your source code and reload (":r") to let the Carp compiler figure out what type goes there.

```clojure
(String.append ?w00t "!") ;; Will generate a type error telling you that the type of '?w00t' is &String
```

### Special forms during evaluation of dynamic code
```clojure
(quote <expression>) ;; Avoid further evaluation of the expression
(and) (or) (not) ;; Logical operators
```

### Dynamic functions
These can only be used at the REPL and during macro evaluation. Here's a subset with some of the most commonly used ones:

```clojure
(car <collection>) ;; Return the first element of a list or array
(cdr <collection>) ;; Return all but the first element of a list or array
(cons <expr> <list>) ;; Add the value of <expr> as the first element the <list>
(cons-last <expr> <list>) ;; Add the value of <expr> as the last element the <list>
(list <expr1> <expr2> ...) ;; Create a list from a series of evaluated expressions
(array <expr1> <expr2> ...) ;; Create an array from a series of evaluated expressions
```

To see all functions available in the `Dynamic` module, enter `(info Dynamic)` at the REPL.

### Modules and Name Lookup
Functions and variables can be stored in modules which are named and can be nested. To use a symbol inside a module
you need to qualify it with the module name, like this: `Float.cos`.

*Using* a module makes it possible to access its members without qualifying them:

```clojure
(use Float)

(defn f []
  (cos 3.2f))
```

If there are several used modules that contain symbols with the same name, the type inferer will try to figure
out which one of the symbols you really mean (based on the types in your code). If it can't, it will display an error.
For example, both the module `String` and `Array` contain a function named 'length'. In the following code it's
possible to see that it's the array version that is needed, and that one will be called:

```clojure
(use String)
(use Array)

(defn f []
  (length [1 2 3 4 5]))
```

In the following example it's not possible to figure out which type is intended:
```clojure
(use String)
(use Array)

(defn f [x]
  (length x))
```

Specifying the type solves this error:
```clojure
(use String)
(use Array)

(defn f [x]
  (String.length x))
```

When you `use` a module, its declarations are brought into the current scope. If you `use` a module in the global scope, all of its declarations are brought into global scope after the call to `use`. Similarly, if you `use` a module in another module's scope, its declarations can be referred to without qualifiers within the scope of the module:

```clojure
(use String)

;; Only the `String` module is used in the global scope,
;; so we can refer to `length` without a module qualifier.
(defn f [x]
  (length x))

(defmodule Foo
  (use Array)
  ;; Since the the `String` module is used in the global scope,
  ;; and the Foo module `use`s `Array`, we again need to qualify calls to `length`
  ;; to disambiguate which declaration we're referring to.
  (defn g [xs]
    (Array.length xs)))
```

Sometimes, it's more convenient to bring a module's declarations into scope only for a limited number of forms. You can do this using the `with` form:

```clojure
(defmodule Foo
  ;; we need to use a module qualifier here,
  ;; since there's no call to `use` in the `Foo` module scope.
  (defn f [x]
    (String.length x))

  ;; Using the `with` form, we can reference the module's declarations
  ;; unqualified in all the forms contained in the `with`'s scope.
  (with String
    (defn g [x]
      (length x))))
```

### Structs
```clojure
(deftype Vector2 [x Int, y Int])

(let [my-pos (Vector2.init 10 20)]
  ...)

;; A 'lens' is automatically generated for each member:
(Vector2.x my-pos) ;; => 10
(Vector2.set-x my-pos 30) ;; => (Vector2 30 20)
(Vector2.set-x! &my-pos 30) ;; => Will update the vector my-pos in place and return ()
(Vector2.update-x my-pos inc) ;; => (Vector2 11 20)
```

### Sumtypes
There are two ways to define `sumtypes`:

**Enumeration:**
```clojure
(deftype MyEnum
  Kind1
  Kind2
  Kind3)
```

**Data:**
```clojure
(deftype (Either a b)
  (Left [a])
  (Right [b]))
```

A Variant can be created with the same syntax as call expression:
```clojure
(MyEnum.Kind1)
(Either.Left 10)
(Either.Right 11)

;; Or use `use` statement
(use Either)
(Left 10)
(Right 11)

(use MyEnum)
(Kind1)
(Kind2)
(Kind3)
```

You can use pattern matching to extract values in a safe way:
```clojure
(defn get [either]
  (match either
    (Either.Left a) a
    (Either.Right b) b))

(with MyEnum
  ;; You can give a generic "otherwise" statement as well
  (match myenum
    (Kind1) (logic1)
    _ (logic-other)))
```

### C Interop
```clojure
(system-include "math.h") ;; compiles to #include <math.h>
(relative-include "math.h") ;; compiles to #include "$carp_file_dir/math.h" where carp_file_dir is the absolute path to the folder containing the invoking .carp file

(register blah (Fn [Int Int] String)) ;; Will register the function 'blah' that takes two Int:s and returns a String
(register pi Double) ;; Will register the global variable 'pi' of type Double

(register blah (Fn [Int Int] String) "exit") ;; Will register the function 'blah' but use the name 'exit' in the emitted C code.

(register-type Apple) ;; Register an opaque C type
(register-type Banana [price Double, size Int]) ;; Register an external C-struct, this will generate getters, setters and updaters.
```

Often type names in C are lowercase (e.g. `size_t`) and just registering them will be problematic since Carp thinks that such variables are generic types.
To be able to interop wich such types, `register-type` takes an optional string after the type name, like this:

```clojure
(register-type SizeT "size_t")
```

This will make the name of the type in Carp code be `SizeT`, while the emitted C code will use `size_t` instead.

[More information on C interop...](./CInterop.md)

### Patterns

Patterns are similar to, but not the same as, Regular Expressions. They were
derived from [Lua](http://lua-users.org/wiki/PatternsTutorial), and are useful
whenever you want to find something within or extract something from strings.

They are simpler than Regular Expressions, as they do not provide alternation.
Nonetheless, they are often very useful and, because they are simpler, also
faster and more predictable.

Here is a little overview of the API:

```clojure
; you can initialize a pattern with a literal or create one from a string
#"[a-z]"
(Pattern.init "[a-z]")

; you can also get a string back from it
(str #"[a-z]")
(prn #"[a-z]")

; you can find things in strings by index
(Pattern.find #"[a-z]" "1234a") ; => 4
(Pattern.find #"[a-z]" "1234")  ; => -1

; also multiple things at once!
(Pattern.find-all #"[a-z]" "1234a b") ; => [4 6]

; matches? checks whether a string matches a pattern
(Pattern.matches? #"(\d+) (\d+)" "  12 13") ; => true

; match-groups returns all match groups of the first match
(Pattern.match-groups #"(\d+) (\d+)" "  12 13") ; => ["12" "13"]

; match-str returns the whole string of the first match
(Pattern.match-str #"(\d+) (\d+)" "  12 13") ; => "12 13"

; global-match gets all match groups of all matches
(Pattern.global-match #"(\d+) (\d+)" "  12 13 14 15") ; => [["12" "13"] ["14" "15"]]

; substitute helps you replace patterns in a string n times
(Pattern.substitute #"sub-me" "sub-me sub-me sub-me" "replaced" 1) ; => "replaced sub-me sub-me"

; if you want to replace every occurrence, use -1
(Pattern.substitute #"sub-me" "sub-me sub-me sub-me" "replaced" -1) ; => "replaced replaced replaced"
```