On Thu, Sep 2, 2010 at 9:31 PM, Andrew U. Frank
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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Here's another way of not quite doing what you want: op :: String -> Either (X A) (X B) op "a" = Left (X "a") op "b" = Right (X "b") which is roughly how I translate the recent discussion about type-level validity certification: http://www.haskell.org/pipermail/haskell-cafe/2010-August/082899.html