Control flow

Gremlin source uses structured control flow. The following program accumulates the integers from its input down to one:

fn main(x: u8) -> u8 {
    let mut n: u8 = x;
    let mut sum: u8 = 0u8;
    while ne(n, 0u8) {
        sum = add(sum, n);
        n = sub(n, 1u8);
    }
    return sum;
}

See the full control-flow example in the repository.

Bindings and loops

Use let for an immutable binding and let mut for a binding that will be reassigned. Assignments keep the original type; names cannot shadow one another.

if, while, and loop introduce structured control flow. break and continue target the nearest loop. Unreachable statements are rejected.

Branches and loops lower carried locals to block parameters in the normalized IR. For the underlying rules, see IR and corpus format.

Source grammar

module = function, {function} ;
function = "fn", identifier, "(", [parameter, {",", parameter}], ")",
           "->", type, "{", {statement}, "}" ;
parameter = identifier, ":", type ;
statement = "let", ["mut"], identifier, ":", type, "=", expression, ";"
          | identifier, "=", expression, ";"
          | "return", expression, ";"
          | "if", expression, "{", {statement}, "}", ["else", "{", {statement}, "}"]
          | "while", expression, "{", {statement}, "}"
          | "loop", "{", {statement}, "}"
          | "break", ";" | "continue", ";" ;
expression = identifier | literal | identifier, "(", [expression, {",", expression}], ")" ;
literal = "true" | "false" | ["-"], decimal_digits, integer_type
        | "0x", hexadecimal_digits, integer_type ;
identifier = (ASCII_letter | "_"), {ASCII_letter | digit | "_"} ;

General CFGs can be expressed in source with explicit block, jump, and branch syntax. That form preserves edge bindings and exact step counts when source is printed from normalized IR.