planetis-m/protocoled

Interface macro for nim

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.

protocoled's Issues

Use macrocache module

By creating const registry of type CacheTable, the nesting can be avoided. But most importantly it'll allow declaring interface implementations across modules https://nim-lang.org/docs/macrocache.html