What you're asking for is essentially a dependent type (something where a type depends on a value). Haskell doesn't support these, but can approximate them with GADTs: {-# LANGUAGE GADTs, EmptyDataDecls, KindSignatures, Rank2Types #-} data A data B -- The data constructors refine the type index data X :: * -> * where A :: X A B :: X B -- We can't return a different type based on the input string (unless you represent the string as some complex GADT that itself refines the output type), so instead we have a pseudo-existential type represented as a polymorphic function parameter. -- This basically says, "if you give me a string and a function that can work on X n for all values of n, I'll give you something of the same type as the return value of that function" op :: String -> (forall n. X n -> r) -> r op "a" f = f A op "b" f = f B If you give a more detailed example of what you need, we might be able to tell you better approaches, though. This rank-2/existential approach is mostly useful for preserving internal (hidden from the end-user) type-level constraints on GADT indices. On Thu, Sep 2, 2010 at 10:31 PM, Andrew U. Frank < frank22@geoinfo.tuwien.ac.at> wrote:
I have a user input (string) and need to select one of two types. depending what the input is. is this possible?
data A data B
data X n = X String
op :: String -> X n op "a" = X "a" :: X A op "b" = X "b" :: X B
this does obviously not compile. is there a way to achieve that the type X A is produced when the input is "a" and X B when the input is "b"?
thank you for help! andrew
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