which is the important hint! the parser used for 'read' depends on the return type, but the existential type _hides_ the internal type which would be needed to select a read parser.
forall e . (MyClass e, Show e, Read e) => MT (e,Int)
the 'Read' there ensures that we only inject types that have a reader, but it doesn't help us select one of the many possible types which have such a reader.
readMT :: ReadPrec MyType readMT = prec 10 $ do Ident "MT" <- lexP parens $ (do { m <- readPrec; return (MT (m::(TipoA,Int))) }) `mplus` (do { m <- readPrec; return (MT (m::(TipoB,Int))) })
The problem is that I was trying to find a way to define the class (MyClass) and not writing a parser for every possible type (or even using their show-representation): I wanted a polymorphic list of types over which I could use the method defined for their class, but, as far as I can get it, this is not possible.
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. 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). 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 instance ReadAsAnyOf () ex where readAsAnyOf ~() = mzero instance (Read t, Show t, MyClass t, ReadAsAnyOf ts MyType) => ReadAsAnyOf (t,ts) MyType where readAsAnyOf ~(t,ts) = r t `mplus` readAsAnyOf ts where r t = do { m <- readPrec; return (MT (m `asTypeOf` (t,0))) } -- 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
Thanks for your kind attention.
you're welcome!-) reading existentials (or gadts, for that matter) is an interesting problem. sometimes too interesting.. claus