Improve error message when instantiating generics that lack a type

[capocasa]

I wish this were a PR but I would require some guidance to where to start differentiating this error condition from others.

Improve error message when instantiating generics that lack a type

Example

import critbits
proc init(): ref CritBitTree =      
  new(result)
var a = init()

Current Output

 Error: cannot instantiate: 'CritBitTree'

Not helpful. Consider a programmer on a deadline after an all nighter at six in the morning. Shouldn't happen, but...

Expected Output

Error: cannot instantiate: 'CritBitTree'
Might the type definition be missing its concrete type, e.g. 'CritBitTree[void]' instead of 'CritBitTree'`?

Sleep deprived programmer now loves the compiler.

Possible Solution

Errors due to lacking concrete type would have to be isolated from other "cannot instantiate" error conditions and this message emitted.

[beef331]

The issue with your suggestion though is that the code is valid. The following is how you'd call it.

import critbits
proc init(): ref CritBitTree =      
  new(result)
var a = init[CritBitTree[void]]()

ref CritBitTree is an implicit generic typeclass which makes the proc definition equivalent to proc init[T: CritBitTree[auto](): ref T = new result.

As such the error message needs to mention that the issue might be the callsite or the definition.

[capocasa]

Thanks!! How about this?

Error: cannot instantiate: 'CritBitTree'
`CritBitTree` is a generic, need concrete type information to instantiate, e.g. 'CritBitTree[void]').

Probably too hard to list all the ways to provide the concrete type n the error message, but at least this clearly states what the problem is.

[beef331]

Yea in the case it's a typeclass lacking parameters I think Cannot instantiate 'CritBitTree', it is a typeclass lacking generic parameter(s) or similar is fine.

[beef331]

A cursory look seems to indicate changing

Nim/compiler/seminst.nim

Lines 57 to 77 in e2e663a

	const
	errCannotInstantiateX = "cannot instantiate: '$1'"
	iterator instantiateGenericParamList(c: PContext, n: PNode, pt: TIdTable): PSym =
	internalAssert c.config, n.kind == nkGenericParams
	for i, a in n.pairs:
	internalAssert c.config, a.kind == nkSym
	var q = a.sym
	if q.typ.kind in {tyTypeDesc, tyGenericParam, tyStatic, tyConcept}+tyTypeClasses:
	let symKind = if q.typ.kind == tyStatic: skConst else: skType
	var s = newSym(symKind, q.name, nextSymId(c.idgen), getCurrOwner(c), q.info)
	s.flags.incl {sfUsed, sfFromGeneric}
	var t = PType(idTableGet(pt, q.typ))
	if t == nil:
	if tfRetType in q.typ.flags:
	# keep the generic type and allow the return type to be bound
	# later by semAsgn in return type inference scenario
	t = q.typ
	else:
	localError(c.config, a.info, errCannotInstantiateX % s.name.s)
	t = errorType(c)

to should suffice in adding the error message desired.

const
  errCannotInstantiateX = "cannot instantiate: '$1'"
  errCannotInstantiateXLackingParams = "cannot instantiate: '$1' it is a typeclass lacking generic parameter(s)."

iterator instantiateGenericParamList(c: PContext, n: PNode, pt: TIdTable): PSym =
  internalAssert c.config, n.kind == nkGenericParams
  for i, a in n.pairs:
    internalAssert c.config, a.kind == nkSym
    var q = a.sym
    if q.typ.kind in {tyTypeDesc, tyGenericParam, tyStatic, tyConcept}+tyTypeClasses:
      let symKind = if q.typ.kind == tyStatic: skConst else: skType
      var s = newSym(symKind, q.name, nextSymId(c.idgen), getCurrOwner(c), q.info)
      s.flags.incl {sfUsed, sfFromGeneric}
      var t = PType(idTableGet(pt, q.typ))
      if t == nil:
        if tfRetType in q.typ.flags:
          # keep the generic type and allow the return type to be bound
          # later by semAsgn in return type inference scenario
          t = q.typ
        else:
          if q.typ.kind == tyCompositeTypeClass:
            localError(c.config, a.info, errCannotInstantiateXLackingParams % s.name.s)
          else:
            localError(c.config, a.info, errCannotInstantiateX % s.name.s)

So have fun with that PR and accompanying tests! 😛