Re: [Haskell-cafe] Strong duck typing / structural subtyping / type class aliases / ??? in Haskell

On Fri, Sep 25, 2009 at 10:55 PM, Casey Hawthorne wrote:

On Fri, 25 Sep 2009 23:25:21 +0200, you wrote:

...

On Fri, Sep 25, 2009 at 8:14 PM, Job Vranish wrote:

...

Supposedly OCaml has an OO feature that does this but I haven't tried it out.

Indeed, OCaml has stuctural polymorphism, it's a wonderful feature.

*# let f myobj = myobj#foo "Hi !";; val f : < foo : string -> 'a; .. > -> 'a = <fun>*

IIRC, there has been work on Template Haskell for structural polymorphism.

Structural subtyping/polymorphism:

Pros: - an object can be coerced to any compatible type, the types do not have to be specified ahead of time, that is at compile time.

Cons: - may be overly permissive; some coercions might not make sense semantically.

I wonder how Haskell will minimize the cons, since it is strongly typed.

I kind of think there's no real problem here. If you say that you can accept any record with a given set of fields, then you have to make sure you make no other assumptions. This is, in principle, no different from passing a speed Double to a function that expects a mass Double (e.g. it may produce garbage for negative values). In both cases a function that requires extra invariants can enforce it by using a newtype that's constructed and manipulated in a way which preserves the extra semantic rules. -- Sebastian Sylvan