Tooling

Compiler — fern

fern [-target arm64|arm64-darwin|x86-64|wasm] [-o OUTPUT] [--run] FILE.fern
fern -fmt [-w | -d] FILE.fern
fern -interp FILE.fern
fern -repl

Common flags

Flag	Default	Meaning
-target	arm64	Backend: arm64, arm64-darwin, x86-64, wasm.
-o	stdout	Output binary path. Without it, assembly prints to stdout.
--run	off	Compile + execute the produced binary. Returns its exit code.
-cc	cc	Linker for native targets (gcc, clang, …).
-qemu	none	Path to a qemu-* binary for cross-arch execution under --run.
-fmt	off	Format the source. -w writes back; -d prints a diff.
-interp	off	Run the AST interpreter (skips codegen entirely).

Formatter

fern -fmt rewrites a file to the canonical style: two-space indent, one statement per line, trailing newline at EOF. Comments survive the round trip in their original position.

fern -fmt -w foo.fern   # rewrite in place
fern -fmt -d foo.fern   # show the diff instead

The formatter is idempotent: fern -fmt | fern -fmt produces identical output.

Language server — fern-lsp

Speaks LSP over stdin/stdout. Spawn it from any editor with a generic LSP client. Features:

• Diagnostics — parser + type-check errors, routed per-file in multi-module programs.
• Hover — types for variables, parameters, fields, methods, cross-module references.
• Goto-definition — across files in workspace mode.
• Completion — locals, params, top-level decls, variants, keywords. Triggered on . and Ctrl/Cmd-Space.
• Signature help — function signatures with active-parameter highlighting.
• Inlay hints — inferred types for var x = ….
• Document symbols — outline view (Cmd-Shift-O in VS Code).
• Semantic tokens — type-aware syntax highlighting.
• Find references + rename — workspace-wide, including method calls / struct fields / enum variants.
• Format on save — runs the formatter via textDocument/formatting.

VS Code extension

Lives at editors/vscode/. Install with:

cd editors/vscode
npm install
npm run compile
npx @vscode/vsce package
code --install-extension fern-vscode-0.1.0.vsix

Set fern.serverPath if fern-lsp isn’t on your $PATH.

REPL

fern -repl
> var x = 7;
> x * 2
14

State persists across lines. Multi-line forms (an if block, function decl, etc.) are one logical input — the REPL reads until braces balance.

Running tests

Fern’s pure-Fern test runner lives in std/test. Tests are ordinary .fern files — run them with -interp (or compile + execute the produced binary):

fern -interp my_test.fern        # AST interpreter
fern my_test.fern -o my_test --run   # compile + run

Output is TAP-13. Exit code is 0 when every case passes and 1 on any failure, so any TAP-aware CI runner (prove, tape, tap-junit) works without further config.

See the Testing tutorial for the authoring shape and assertion catalogue.