Core Concept

In Letlang, values are not assigned a single type. Instead, each type determines which values it contains.

For example:

• 42 is an int, a number, and the singleton type 42
• "hello" is a string and the singleton type "hello"

We can consider a type definition to be some kind of function which returns true or false wether the value belongs to that type.

Primitive types

The Letlang runtime MUST provide the following types:

• bool which contains the values true and false
• int which contains every 64-bits floating-point number with the fractional part equal to 0
• number which contains every 64-bits floating-point number
• string which contains every UTF-8 encoded strings
• atom which contains every atom

NB: An atom is a developer-defined symbol, for example: @ok, @error, @hello_world.

Additionally, the Letlang runtime MUST provide the following types:

• pid representing Letlang process identifiers
• file representing file descriptors
• socket representing socket descriptors

Those types MUST NOT be forgeable by the developer, only the Letlang runtime should be able to create values of those types.

Literal values MUST match the following syntax:

literal
  = literal_bool
  / literal_number
  / literal_string
  / literal_atom

literal_bool
  = "true"
  / "false"

literal_number
  = "0b" "_"* [01] [_01]*
  / "0o" "_"* [0-7] ["_" 0-7]*
  / [0-9][_0-9]*
  / "0x" "_"* [0-9a-fA-F][_0-9a-fA-F]*
  / (([0-9]+ "."? [0-9]*) / ("." [0-9]+)) (([eE][+-]?)? [0-9]+)?

literal_string
  = "\"" ([^"] / "\\\"")* "\""

literal_atom
  = "@" ( ("'" ([^'] / "\\'")+ "'") / ([_a-zA-Z][_a-zA-Z0-9]*))

Container types

The Letlang runtime MUST provide the following container types:

• list<T>, a generic type representing a list whose elements are of type T, for example: [1, 2, 3] is a list<int>
• tuples, for example: (int, string) contains values such as (42, "hello")
• structures, for example: {foo: string} contains values such as {foo: "bar"}

The Letlang runtime MUST NOT provide a tuple or struct type containing “every tuples” or “every structures”.

Singleton types

Every value MUST be its own singleton type, for example:

• 42 is a singleton type containing only the value 42
• "foo" is a singleton type containing only the value "foo"
• (@ok, 42) is a singleton type containing only the value (@ok, 42)

Combining types

The Letlang runtime MUST allow types to be combined using | (or), & (and) and/or ! (not).

For example:

• int | string is a type containing all integers and all strings
• !int is a type containing all values, except integers
• number & !int is a type containing all numbers except integers

This can be used to define tagged enumerations, such as:

• (@ok, int) | (@error, string), similar to Rust’s Result type
• @none | (@some, int), similar to Rust’s Option type

Functional types

Every function is a value whose type is its signature, for example:

• func foo(bar: string) -> @ok { ... } belongs to the type func[(string) -> @ok]
• func add(a: int, b: int) -> int { ... } belongs to the type func[(int, int) -> int]

Referencing types

A type reference is defined using the following syntax:

typeref
  = typeref_val
  / typeref_name
  / typeref_struct
  / typeref_tuple
  / typeref_func
  / typeref_oneof
  / typeref_allof
  / typeref_not

typeref_val
  = literal

typeref_name
  = name:identifier
    type_params:("<" identifier ("," identifier)* ","? ">")?

typeref_struct
  = "{" members:(typeref_struct_member ("," typeref_struct_member)* ","?)? "}"
typeref_struct_member
  = field:identifier ":" type:typeref

typeref_tuple
  = "(" members:(typeref ("," typeref)* ","?)? ")"

typeref_func
  = "func" "[" "(" params:(typeref ("," typeref)* ","?)? ")" "->" return_type:typeref "]"

typeref_oneof
  = typeref "|" typeref

typeref_allof
  = typeref "&" typeref

typeref_not
  = "!" typeref

identifier
  = [_a-zA-Z][_0-9a-zA-Z]*

Defining types

A custom type is defined using a class:

• it may have a public visibility (modules importing this module may reference it)
• it must have a constructor parameter (defining the structure of the values it contains)
• it may have a predicate to further restrain what values it contains

The type’s predicate (if present) MUST return a bool.

The class is defined using the following syntax:

class_decl_statement
  = "pub"? "class" name:identifier
    type_params:("<" identifier ("," identifier)* ","? ">")?
    "(" cons_param:(variable:identifier ":" typename:typeref) ")"
    (";" / predicate:("{" proposition+ "}"))

proposition
  = proposition_let
  / proposition_expr

NB: proposition_let and proposition_expr rules will be defined later.

The value of the last proposition of the class’s predicate evaluates to is the return value of the predicate.

Example:

module example;

class even(n: int) {
  n % 2 = 0;
}

class odd(n: int & !even);

Type Checking

The Letlang runtime MUST perform type checking at the function’s boundaries:

• type checking the function arguments before its invocation
• type checking the function’s return value after its invocation

The Letlang runtime MUST perform type checking when binding a value to a variable.

The Letlang runtime MUST perform type checking when requested manually by the developer.