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Functions

This page covers function definitions, calling conventions, signatures, and advanced patterns in TypR.

Defining typed functions​

The fn keyword defines a typed function. Parameter types use : type, and the return type appears after the closing parenthesis:

let add <- fn(a: int, b: int): int {
    a + b
};

print(add(5, 3));
caution

fn(...) always requires a return type — omitting it triggers an explicit panic ("You forgot to specify the function return type").

Default parameters​

Default values are supported for the final parameter(s):

let greet <- fn(name: char, greeting: char = "Hello"): char {
    greeting
};

greet("World");        # "Hello"
greet("World", "Hi");  # "Hi"

Variadic functions​

let sum_all <- fn(...xs: int): int {
    sum(xs)
};

print(sum_all(1, 2, 3));

The ... prefix makes a parameter accept any number of arguments.

Calling conventions​

TypR supports three equivalent ways to call a function, thanks to the uniform function call syntax (UFCS):

# Classic call
print(add(5, 3));

# Pipe syntax (|>)
(5) |> add(3)
    |> print();

# Method call syntax (.)
(5).add(3)
   .print();

All three styles produce the same result. The first argument can be "pulled out" as the receiver in pipe or method-call notation — making operation chaining readable and natural.

Lambdas​

Lambdas use \ without type annotations:

let sq <- \(x) x * x;
info

\(...) (lambda) has no declared parameter types or return type. Use fn(...) when types are needed.

Partial application​

The \ symbol also supports partial application of functions:

# --- setup ---
let add <- fn(a: int, b: int): int { a + b };
type Point <- list { x: int, y: int };
# -------------

let add5 <- \add(a = 5);                         # partial application of a function
let origin <- \Point:{ x = 0, y = 0 };           # partial application of a record constructor

print(add5(3));
note

Lambda vs partial application: both use \, disambiguating by what follows — \( → lambda, \identifier( → partial application. PartialApp is desugared into Lang::Function during type-checking and never reaches transpilation.

Higher-order functions​

Functions are first-class values in TypR. They can be passed as arguments and returned as values:

type Function <- (int) -> bool;

let function0 <- fn(a: int): bool {
    true
};

The type system tracks function types using (T1, T2) -> T3 syntax, ensuring composition and callbacks are type-safe.

Closures​

Functions can capture variables from their surrounding environment. The type system ensures captured variables and returned functions remain type-safe:

let make_adder <- fn(n: int): (int) -> int {
    fn(x: int): int { x + n }
};

let add5 <- make_adder(5);
add5(3);   # 8

Signatures: typing existing R functions​

By default, most base R functions accept Any and return Empty. The @ annotation declares the type of an existing R function without modifying it:

@toupper: (char) -> char;

toupper("Hi"); # now takes char, returns char
# toupper(7);  # would now fail at compile time

See Signatures, @extern & Foreign for overloading, @extern, and @importFrom.

Interfaces: polymorphic functions​

Interfaces enable ad-hoc polymorphism — write functions that work across multiple types:

@paste: (Any, Any) -> char;

type Viewable <- interface {
    view: (Self) -> char
};

let double <- fn(a: Viewable): char {
    paste(view(a), view(a))
};

let view <- fn(a: bool): char { "bool" };

true.double();  # works because bool implements Viewable

See Interfaces & Structural Validation for details.