Too strong type bindings with multiple generic parameters

[dcurrie]

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When a generic type has multiple generic parameters, it's not possible to instantiate types that share the same concrete type for the first generic parameter.

Example

  import hashes, sets, tables
  
  type 
    DEdge*[K, V] = ref object
      fm*: DNode[K]
      to*: DNode[K]
      weight*: V
  
    DNode*[K, V] = ref object
      key*: K
  
    DGraph*[K, V] = object
      # A directed graph of nodes and directed edges
      nodes: Table[K, DNode[K, V]]
  
  proc add_edge*[K, V](graph: var DGraph[K, V], fm: K, to: K, weight: V = V(1)): DEdge[K, V] =
    var n1 = DNode[K, V](key: fm)
    var n2 = DNode[K, V](key: to)
    result = DEdge[K, V](fm: n1, to: n2, weight: weight)
  
  when true:
  
    block:
      var w = DGraph[int,int]()
      let _ = w.add_edge(0, 1, 10)
      echo w
  
    block:
      var g = DGraph[int,float]()
      let _ = g.add_edge(3,4,3.5)
      echo g
  
  when defined(test2):
  
    block:
      var w = DGraph[int,int]()
      let _ = w.add_edge(0, 1, 10)
      echo w
  
    block:
      var g = DGraph[uint,float]()
      let _ = g.add_edge(3u,4u,3.5)
      echo g

Current Output

Building with -d:test1 yields:

Error: type mismatch: got <DNode[system.int, system.float]> but expected 'DNode[system.int, system.int]'

Expected Output

Building with -d:test2 works as expected.

Possible Solution

?

Additional Information

Per @mratsim : Nim type bindings rules are in-depth but unlike the examples given there your example is expected to work with the current Nim code.

https://forum.nim-lang.org/t/5640

% nim -v
Nim Compiler Version 1.0.4 [MacOSX: amd64]
Compiled at 2019-11-27
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: c8998c498f5e2a0874846eb31309e1d1630faca6
active boot switches: -d:release