I'd like to be able to use MT to build a list like: [MT (T1a,1), MT (T1b,3)] And I'd like to read str with: read $ show str
Substituting return (m) with return (MT m) leads to error messages like: Ambiguous type variable `e' in the constraints
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.
readMT :: ReadPrec MyType readMT = prec 10 $ do Ident "MT" <- lexP parens $ do m <- readPrec return (m)
if your hidden types have distinguishable 'show'-representations, you could write your own typecase like this (making use of the fact that 'read' parsers with incorrect type will fail, and that the internal type can be hidden after parsing) 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))) }) *Test> read (show [MT (T1a,1),MT (T1b,3)]) :: [MyType] [MT (T1a,1),MT (T1b,3)] (if necessary, you could have 'show' embed a type representation for the hidden type, and dispatch on that representation in 'read') claus