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5819 :: how to evaluate a code block in a different context in a macro

• egx (orginal) [2020-01-16T18:11:15+01:00] view original

  Hi,

  Is it possible to evaluate function body, or some other code block, in a different context, e.g. consider this simple repro:

  import macros
  
  # this macro evaluates function body in a different context
  # and prints result at compile time
  macro eval1(fun: untyped): untyped  =
    fun.expectKind nnkProcDef
    let body = fun.body
    let stmt = body[0]
    echo stmt.treeRepr
    let e =
      quote do:
        let x = 42
       `stmt`
    echo e  # i'd like to see value 48 printed at compile time,
  
  
  proc stmt1 {.eval1.}=
    2*x
  Run

  What I'd like is to evaluate the function body in a different context inside the macro. I seem unable to find out how to accomplish this.

  Thanks

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• mratsim (orginal) [2020-01-16T22:59:59+01:00] view original

  I'm not exactly sure what you mean but I assumed that:

  • by context you mean scope, if so you can use a block statement
  • for compile-tie printing you can use static
  • You wanted to see 84 printed and not 48

  import macros
  
  # this macro evaluates function body in a different context
  # and prints result at compile time
  macro eval1(fun: untyped): untyped  =
    fun.expectKind nnkProcDef
    let body = fun.body
    let stmt = body[0]
    echo stmt.treeRepr
    let x = ident"x" # Avoid capturing an inexisting ident x
    result =
      quote do:
        block:    # Create a new scope
          static: # i'd like to see value 48 printed at compile time,
            let `x` = 42
            echo `stmt`
  
  proc stmt1 {.eval1.}=
    2*x
  
  # Returns 84
  Run

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