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zig-repl

An expression-first REPL for Zig.

It evaluates input with a faithful port of the compiler's own semantic analysis, so results match what zig folds at comptime.

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How it works

Input is parsed by Zig's own parser and lowered to ZIR by std.zig.AstGen; the same front end the compiler uses.

Evaluation is a port of the compiler's Sema, InternPool, Type, and Value, run comptime-eager: every expression folds to a concrete value, with session state persisting across lines.

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Run it

zig build run     # the terminal REPL
zig build wasm    # the wasm REPL + web explorer -> zig-out/web
zig build test    # compliance and unit tests (--fuzz to drive the coverage-guided fuzzer)

A hosted build runs in the browser at brianferri.github.io/zig_repl.

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Commands

• :help list every command with its summary
• :dump <expr> show the AST and tree-walked ZIR for an expression
• :theme [name] show or switch the prompt theme (zig, catppuccin, adwaita-dark)
• :terminal show the active terminal's detected capabilities
• :clear wipe the output log (web)
• :quit exit the REPL

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What it evaluates

Values & aggregates

Integers, floats, bools, strings; structs, tuples, enums, unions; arrays, slices, and vectors.

Pointers & wrappers

Pointers, optionals, and error unions, with the pointer-cast family (@ptrCast, @alignCast, @constCast, @volatileCast).

Control flow

if / switch / while / for, plus catch, try, and error-union capture.

Types & reflection

@TypeOf, @typeInfo, @Type, @sizeOf / @alignOf / @offsetOf, and @import of std, root, and local modules.

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Web explorer

The wasm frontend is two views over the same core:

• a REPL with persistent session state, and
• an explorer that renders the AST and the tree-walked ZIR for any input side by side, so you can watch the front end lower your code.

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