Mere REPL — written in Mere, running in the browser

Paste a Mere expression or a full file on the left and press Run. The Wasm module on this page tokenizes, parses, and evaluates your input — all written in Mere itself, no server round-trip. The companion page self-host fmt covers the format half of the pipeline; this one closes the loop.

Source: eval.mere (tree-walking interpreter), parser.mere, lexer.mere, bridged by selfhost-repl.mere. The shared AST lives in contrib/parser/ast.mere.

Result

(press Run to evaluate)

print output

Pipeline

Four contrib libraries combine into the Wasm running on this page:

lexer.mere — source → tokens
parser.mere — tokens → AST
eval.mere — AST → value (this stage)
selfhost-repl.mere — DOM glue (textarea → parse_and_eval → output)

eval.mere covers literals, the binary / comparison / logical operator groups, if, closures with captured env, let and match with the full pattern set, let rec with mutual recursion via the VRecBinding placeholder trick (pure-functional, no ref), constructors, tuples, lists (with [1, 2, 3] reconstruction over Cons / Nil), and records (literal / field access / update / pattern match). The host-provided builtin set is currently just print and show — most useful work flows through the operator-level paths instead.

Limitations

Errors abort the Wasm instance. If your input triggers a parse error, unbound variable, or pattern mismatch, the message goes to print output and the Wasm traps; reload the page to recover. (The OCaml side uses exceptions for this; the self-host stays simple and uses fail which aborts.)
Builtin set is minimal. Only print and show are wired through as host functions. Multi-arg builtins (str_concat, list_map, …) need partial application machinery on VBuiltin — a follow-up slice. Most expressions flow through EBin / ECmp / EApp with user-defined closures, so this constrains less than it sounds.
No types. The evaluator runs untyped. Type annotations parse fine and are ignored at eval time; type decls register no runtime info. A self-host typer is §S2 Phase 52, still on paper.

Why this matters

Phase 51 closes §S2.A — the self-host evaluator. Combined with §S1 (self-host parser + fmt, Phase 49 + 50), Mere can now parse, format, AND evaluate its own source code, in the browser, with no server round-trip.