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FS-1334-elif-preprocessor-directive.md

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F# RFC FS-1334 - Add #elif Preprocessor Directive

Summary

Add #elif to F# conditional compilation so multiple mutually exclusive branches can be written linearly (#if ... #elif ... #elif ... #else ... #endif) instead of nesting or repeating #if blocks. Aligns F# with C# and other languages; improves readability.

Motivation

Without #elif, developers either:

  • Nest #if inside #else (deep indentation, harder to scan), or
  • Write separate #if blocks plus a final negated catch-all (#if !A && !B && !C), which is verbose and error-prone.
  • C# parity

#elif removes redundancy, reduces logical mistakes, and eases cross-language sharing.

Design

Grammar change:

#if <cond>
  group
#elif <cond>
  group
... (0+ more #elif)
#else
  group
#endif

First matching condition’s group is included; at most one group is compiled. #elif must follow an #if and precede an optional #else. No change to symbol definition semantics.

Example #1

#if WIN64
let path = "/library/x64/runtime.dll"
#elif WIN86
let path = "/library/x86/runtime.dll"
#elif MAC
let path = "/library/iOS/runtime-osx.dll"
#else
let path = "/library/unix/runtime.dll"
#endif

Example #2

module test =
 // Should evaluate to x = 3
 let x = 
#if false
   1
#elif false
   2     
#elif (true || false)
   3
#elif true
   4
#else
   5
#endif

Spec Changes

Update conditional compilation section to:

  • Add { #elif pp-expression group } to grammar.
  • Document ordering and “first true wins” semantics.
  • State errors: #elif after #else, #elif without #if, missing condition.

Drawbacks

Small parser/editor updates (fantomas, etc); possibility of long chains (already possible).

Alternatives

Keep nesting; use multiple guarded blocks; introduce more complex compile-time constructs (unnecessary).

Prior Art

VB.NET, C/C++, C#, Swift, Objective-C all support #elif (or equivalent). Aligns F# with ecosystem norms.

Compatibility

Non-breaking. Older compilers will error on #elif. No IL or FSharp.Core changes.

Diagnostics

Errors for:

  • #elif without preceding #if
  • #elif after #else
  • Missing condition
  • Unbalanced directives

Tooling

Add keyword highlighting (or darken un-used branch), folding across entire block, completion for #elif. No runtime tooling differences. Tools like Fantomas must update their logic that evaluates the #if conditions to handle #elif.

Performance

Negligible; linear evaluation of conditions. No impact on generated code beyond selected branch.

Scaling

Typical branch count ≤ 8; acceptable upper bound ≈ 50; linear processing.

Migration Example

Before:

#if A
let mode = 1
#else
#if B
let mode = 2
#else
let mode = 3
#endif
#endif

After:

#if A
let mode = 1
#elif B
let mode = 2
#else
let mode = 3
#endif