I would recommend using http://hackage.haskell.org/package/base-4.8.2.0/docs/Prelude.html#v:asTypeOf And passing a dummy variable of the correct type. -Will

On Mar 28, 2016, at 12:53, Martijn Rijkeboer wrote:

Hi,

I'm writing the following Hspec + QuickCheck test for some type:

context "Type1" $ it "decode inverses encode" $ property $ \x -> (decode . encode) x == Right (x::Type1)

However, I have a number of these types, and I could write the same test for every type, but that isn't very DRY. Therefore I would like to put the test in a function and call that function for every type. Something like the following:

typeTest name = do context name $ it "decode inverses encode" $ property $ \x -> (decode . encode) x == Right x

Unfortunately this doesn't work, since QuickCheck doesn't know which Arbitrary instance to use. How can I pass the type, e.g. x::Type1, to this function so QuickCheck knows what Arbitrary instance to use?

Kind regards,

Martijn Rijkeboer

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Martijn Rijkeboer writes:

Unfortunately this doesn't work, since QuickCheck doesn't know which Arbitrary instance to use. How can I pass the type, e.g. x::Type1, to this function so QuickCheck knows what Arbitrary instance to use?

For example: {-# LANGUAGE ScopedTypeVariables #-} import Test.QuickCheck import Test.Hspec import Data.Serialize import Data.Proxy typeTest :: forall a. (Show a, Eq a, Arbitrary a, Serialize a) => String -> Proxy a -> SpecWith () typeTest name _ = context name $ it "decode inverses encode" $ property $ \(x :: a) -> (decode . encode) x == Right x main :: IO () main = hspec $ typeTest "foo" (Proxy :: Proxy Int) The use of Proxy conveys the needed type information, as Will had also suggested. -- John Wiegley GPG fingerprint = 4710 CF98 AF9B 327B B80F http://newartisans.com 60E1 46C4 BD1A 7AC1 4BA2