scf - Standalone Nim Source Code Filter

A standalone implementation of Nim's stdtmpl source code filter, extracted from the compiler for use at compile-time in macro contexts.

Why

Nim's source code filters (SCF) like #? stdtmpl are processed by the compiler before parsing. This makes them impossible to use with parseStmt() in macros - the filter syntax isn't valid Nim.

This package provides the same transformation as a pure function, allowing:

const scfSource = staticRead("template.nimf")
const nimSource = filterStdTmplAuto(scfSource)  # transform at compile-time
let ast = parseStmt(nimSource)  # now it's valid Nim

Usage

As a library

import scf

# Auto-detect options from #? directive
let output = filterStdTmplAuto(input)

# Or specify options manually
let output = filterStdTmpl(input,
  subsChar = '$',
  nimDirective = '#',
  emit = "result.add",
  conc = " & ",
  toStr = "$")

As CLI

scf template.nimf > output.nim
cat template.nimf | scf > output.nim

Supported Features

All stdtmpl features are supported:

• $var and ${expr} substitution
• $$ escape for literal $
• Control flow: if/elif/else/end, for/end, while/end, try/except/finally/end
• Declarations: let, var, const, type, proc, template, macro, etc.
• Custom parameters: subsChar, metaChar, emit, conc, tostring

Whitespace Behavior

The output is functionally equivalent to the Nim compiler's filter, not byte-identical. Differences:

1. String literal continuation indentation - My output uses 4-space continuation; compiler varies. Both produce identical runtime output.

2. Trailing content - Minor differences in how trailing newlines are grouped into result.add() calls.

These differences are all inside string literals or inside parentheses, so they don't affect Nim's indentation-based parsing. The generated code compiles and runs identically.

Testing

nim c -r tests/test_scf.nim

18 tests covering all stdtmpl features, verified against examples from Nim's official documentation.

License

MIT (same as Nim)