partial type signatures/annotations/declarations..

[Claus Reinke]

when trying to look up the state of this proposal, I noticed:

 - there seem to be two parallel versions (probably signatures
    is an older form, and all references ought to point to annotations
    instead?):

    http://hackage.haskell.org/trac/haskell-prime/wiki/HaskellExtensions
    points to:
    http://hackage.haskell.org/trac/haskell-prime/wiki/PartialTypeSigs
    http://hackage.haskell.org/trac/haskell-prime/ticket/40

    whereas

    http://hackage.haskell.org/trac/haskell-prime/report/9
    points to:
    http://hackage.haskell.org/trac/haskell-prime/wiki/PartialTypeAnnotations
    http://hackage.haskell.org/trac/haskell-prime/ticket/86

- the proposal focusses on using "_" as a place holder for unspecified 
    parts of a type or context, without discussing the alternative I'd favour:

    instead of introducing holes in types and contexts to leave
    parts of a declaration unspecified, why not use type subsumption?

    the idea being, that instead of declaring the precise type, one
    would declare an upper bound on the type, and the precise type
    inferred needs to be subsumed by the one declared. type and 
    context syntax would remain unchanged, but in addition to "::"
    for precise type annotations, there'd be ":::" (or "::<", or whatever)
    to indicate the difference in intended semantics.

    pro: would easily allow for omission of type details or parts
            of context (a type with more context, or with more specific
            type components, is subsumed by the declaration)

    cons: as long as we only specify an upper bound, the inferred
            type could be more specific than we'd like (we can't say
            that we don't want context, or that some type variable
            must not be instantiated)

cheers,
claus