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
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