support `when` in case statements

[timotheecour]

proposal

allow when in case statements

example

type Foo = enum f0 f1
proc main()=
  var f = f0
  case f
  of f0: echo "ok0"
  of f1: echo "ok1"
  when defined(case2_1):
    of f2:
      echo "ok2"
  elif defined case2_2:
    of f2b, f2c:
      echo "ok2b"
  elif defined case2_3:
    else: # else is allowed too
      echo "ok3"
main()

likewise with case objects:

type Foo = enum f0 f1
type Bar = object
  case f: Foo
  of f0: v0: int
  of f1: v1: int
  when not defined(js):
    of f3: v3: int

implementation

inside a when statement (or elif branch following a when statement), a of or else shall be attached to preceding case statement.

related

• https://github.com/nim-lang/RFCs/issues/190 is related and would work well in combination with this RFC but both are independently useful
• I suspect this RFC is less impactful than https://github.com/nim-lang/RFCs/issues/190 in terms of making existing macros (macros manipulating case..of would now need to handle the case of when branches to be 100% correct, but these are likely less frequent than macros handling enum's)

[mratsim]

Is there a reason for closing this RFC? Change of mind?

[timotheecour]

Wasnt complete yet, I've writing a library solution for this and will compare to a compiler solution, more details soon