String Interpolation

Double-quoted string literals in Forge support interpolation: embedding arbitrary expressions inside { and } delimiters. The expressions are evaluated at runtime and their results are converted to string representations and spliced into the surrounding text.

Syntax

" ... {expression} ... "

Any valid Forge expression may appear between the braces.

Basic Usage

let name = "Forge"
say "Hello, {name}!"           // "Hello, Forge!"

let x = 10
let y = 20
say "{x} + {y} = {x + y}"     // "10 + 20 = 30"

Arbitrary Expressions

The interpolated expression is not limited to identifiers. Any expression that produces a value is permitted, including function calls, arithmetic, method calls, and field access.

say "length: {len([1, 2, 3])}"             // "length: 3"
say "upper: {"hello".upper()}"             // "upper: HELLO"
say "sum: {1 + 2 + 3}"                     // "sum: 6"
say "type: {typeof(42)}"                    // "type: int"

Value Conversion

Interpolated values are converted to their string representation using the same rules as the str() builtin:

Escape Sequences

The following escape sequences are recognized inside double-quoted strings:

To include a literal { character without triggering interpolation, escape it:

say "use \{braces\} for interpolation"
// Output: use {braces} for interpolation

Nesting

Interpolated expressions may themselves contain string literals with interpolation, though this is discouraged for readability:

let items = ["a", "b", "c"]
say "result: {join(items, ", ")}"

Non-Interpolated Strings

Single-quoted strings do not support interpolation. Use single quotes when the string contains braces that should be treated literally:

say 'no {interpolation} here'
// Output: no {interpolation} here

Empty Interpolation

An empty interpolation {} is not valid and produces a parse error.

Implementation Notes

String interpolation is parsed into a StringInterp AST node containing a sequence of StringPart elements, each being either a Literal (raw text) or an Expr (an evaluated expression). At runtime, all parts are evaluated and concatenated to produce the final string value.