As this problem requires type to depend on runtime value, you need singletons. data Format = Hdf5 | Cbf data SFormat a where -- this is called a singleton type SHdf5 :: SFormat Hdf5 SCbf :: SFormat Cbf data A (t::Format) = A String data SomeA where SomeA :: SFormat f -> A f -> SomeA check :: String -> SomeA check "foo" = SomeA SHdf5 (A "foo") check "bar" = SomeA SCbf (A "bar") you can recover the type of A be pattern-matching: someFunc :: SomeA -> A Hdf5 someFunc (SomeA SHdf5 a) = a -- a has type A Hdf5 here, equation typechecks someFunc (SomeA SCbf a) = a -- a has type A SCbf here, this is a type error You will need KindSignatures, DataKinds and GADTs language extensions. With some effort you can probably add Unchecked type tag to the picture if you don't want to use just Strings for A Unchecked. One downside of this method is that you need to enumerate all the possible tags twice. There is a singletons package [1] that can automatically generate SFormat for you. [1] http://hackage.haskell.org/package/singletons On 03/10/2018, Francesco Ariis <fa-ml@ariis.it> wrote:
On Tue, Oct 02, 2018 at 05:04:42PM +0000, PICCA Frederic-Emmanuel wrote:
I believe you can do this with GADTs [1]
I can create different constructors for the different types. but how can I create a function which return different type ?
Mhhh I tried to come up with an example (GADTs, ExistentialQuantification, etc.) and failed...
This is an interesting problem and one that could interest many people; please post your question on -cafe too (with a minimal .hs, it always helps); I am curious on how they will approach the problem
-F
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