Protocoled — an interface macro for Nim

About

This nimble package contains the protocol macro for easily implementing interfaces in Nim.

The protocol macro

Example:

import protocoled

protocol PExpr:
   proc eval(e): int

   impl PLiteral:
      var x: int
      proc eval(e): int = e.x
      proc newLit(x: int): PLiteral =
         result = PLiteral(x: x)

   impl PPlusExpr:
      var a, b: PExpr
      proc eval(e): int = eval(e.a) + eval(e.b)
      proc newPlus(a, b: PExpr): PPlusExpr =
         result = PPlusExpr(a: a, b: b)

Notice the typeless parameter e, the macro takes care of assigning it the proper type. Then it is translated roughly into this code:

type
   PExpr = ref object of RootObj ## abstract base class for an expression
      evalImpl: proc(e: PExpr): int {.nimcall.}
   PLiteral = ref object of PExpr
      x: int
   PPlusExpr = ref object of PExpr
      a, b: PExpr

proc eval(e: PExpr): int =
   assert e.evalImpl != nil
   e.evalImpl(e)

proc evalLit(e: PExpr): int = PLiteral(e).x
proc evalPlus(e: PExpr): int = eval(PPlusExpr(e).a) + eval(PPlusExpr(e).b)

proc newLit(x: int): PLiteral = PLiteral(evalImpl: evalLit, x: x)
proc newPlus(a, b: PExpr): PPlusExpr = PPlusExpr(evalImpl: evalPlus, a: a, b: b)

Known quirks

• You need to separate the self parameter from the rest with a semicolon ;.
• The export marker * is using infix notation like so: impl *Student.
• In the constructor proc, implicit return of the last expression is not supported.

License

This library is distributed under the MIT license. For more information see copying.txt.