[Haskell-cafe] reading existential types

[Andrea Rossato]

On Mon, Jul 09, 2007 at 09:41:32PM +0100, Claus Reinke wrote:
>  i'm not sure i understand the problem correctly, but note that the branches
>  in 'readMT' have identical implementations, the only difficulty is 
>  instantiating
>  them at different hidden types, so that they try the appropriate 'Read' 
>  instances for those types. there's no need for different parsers beyond the 
>  'Read' instances for every possible type.

This is now clear.

>  hiding concrete types in existentials sometimes only defers problems
>  instead of solving them, but exposing class interfaces instead of types is a 
>  useful way to mitigate that effect. it just so happens that this particular 
>  problem, reading an existential type, slightly exceeds that pattern, as 
>  'read' needs to know the hidden type to do its job ('read' does not 
>  determine the type from the input form, but uses the type to determine what 
>  form.the input should have). 

That's exactly what I had in mind. But I ignored the specific problem
of reading: "'read' uses the type", and the type is hidden.. 

>  a workaround is to try to read all possible types, then hide the type again 
>  once a match is found. the main disadvantage of this method is that we need 
>  a list of all the types that could possibly be hidden
>  in 'MyType' (or at least a list of all the types that we expect to
>  find hidden in 'MyType' when we read it).
> 
>  we can, however, abstract out that list of types, and write a general
>  type-level recursion to try reading every type in such a list:
> 
>   class ReadAsAnyOf ts ex -- read an existential as any of hidden types ts
>     where readAsAnyOf :: ts -> ReadPrec ex
> 
[...]
>   -- a list of hidden types
>   hidden = undefined :: (TipoA,(TipoB,()))
> 
>   readMT :: ReadPrec MyType
>   readMT = prec 10 $ do
>              Ident "MT" <- lexP
>              parens $ readAsAnyOf hidden -- r T1a `mplus` r T1b


This is a nice work around indeed and could be suitable for my
specific problem.

Thank you very much: this thread perfectly clarified the issue I was
facing. Most kind of you.

BTW, I have the feeling that existential types should be used with
extreme care since they open up great possibilities of really hidden
bugs: in other word, it seems to me they could be use to just trick
the type checker. Sort of scary. Am I right?

Thanks
Andrea