Won't show include all the components? I wanted to include only the type name. I could probably get away with bundling all the contents into the error string but it would be nice to know if there is a standard way to get type names like this. On Sun, Jan 12, 2014 at 2:33 PM, fa-ml wrote:

On Sun, Jan 12, 2014 at 02:04:40PM +0000, Gareth Morgan wrote:

...

I have a sum type, when the input is type A I want to process it, when it is type B I want to return MyError "Expected type A but actually got type B". However I also have C,D,E,F, etc which should also return similar errors so don't want to hardcode this string.

Would pattern matching be enough for you? i.e. (very crude):

data Test = Alfa | Beta | Gamma deriving (Show)

testAlfa Alfa = undefined -- put your process function here testAlfa o = error $ "wasn't expecting " ++ show o

main = testAlfa Beta

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Data.Typeable with Data.Data worked. Thank you. Using Typeable alone was only giving me the type of the whole type. Data.Data had the toConstr function needed to give me the particular constructor name. For reference if anyone is interested in the solution: import Data.Typeable import Data.Data data A = A | B | C deriving (Typeable, Data) typeName :: A -> String typeName = show . toConstr I needed to add DeriveDataTypeable extension to my cabal file On Sun, Jan 12, 2014 at 2:53 PM, Brandon Allbery wrote:

On Sun, Jan 12, 2014 at 9:47 AM, Gareth Morgan wrote:

...

Won't show include all the components? I wanted to include only the type name.

Types only exist at compile time normally. But if you add a Typeable constraint (see Data.Typeable and the DeriveDataTypeable extension) you can get type names at runtime.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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