compileTime

compileTime pragma

The compileTime pragma is used to mark a proc or variable to be used only during compile-time execution. No code will be generated for it. Compile-time procs are useful as helpers for macros. Since version 0.12.0 of the language, a proc that uses system.NimNode within its parameter types is implicitly declared `compileTime`:

proc astHelper(n: NimNode): NimNode =
result = n

Is the same as:

proc astHelper(n: NimNode): NimNode {.compileTime.} =
result = n

compileTime variables are available at runtime too. This simplifies certain idioms where variables are filled at compile-time (for example, lookup tables) but accessed at runtime:

import std/macros

var nameToProc {.compileTime.}: seq[(string, proc (): string {.nimcall.})]

macro registerProc(p: untyped): untyped =
  result = newTree(nnkStmtList, p)

  let procName = p[0]
  let procNameAsStr = $p[0]
  result.add quote do:
    nameToProc.add((`procNameAsStr`, `procName`))

proc foo: string {.registerProc.} = "foo"
proc bar: string {.registerProc.} = "bar"
proc baz: string {.registerProc.} = "baz"

doAssert nameToProc[2][1]() == "baz"