[Haskell-cafe] Subsets and supersets

[Yves Parès]

> your subtype relation says A -> B is a subtype of C ->D whenever
> A is a supertype of C and B is a subtype of D, then checking subtyping is
undecidable.

In fashion terms: operator (->) is contravariant in its first argument and
covariant in its second.
;)

2011/3/16 Brandon Moore <brandon_m_moore at yahoo.com>

> You want polymorphic variants. Check out O'Caml, or MLPolyR.
>
> Subtyping is not very compatible with first-class functions. If you have
> subtype-bounded polymorphism
> ("forall A a subtype of T, ..."), and your subtype relation says A -> B is
> a
> subtype of C ->D whenever
> A is a supertype of C and B is a subtype of D, then checking subtyping is
> undecidable.
>
> I might as well mention here that in any OO language with protected methods
> or
> members, a
> subclass is not a subtype in any behavioral sense - and terminal coalgebras
> have
> so little structure
> that just working in terms of the interface doesn't seem too useful.
>
> Brandon.
>
>
>
>
>
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