Hi Alexander: You DO know the concrete type of b in the context of get: its necessarily a String. Therefore, get :: a -> Value String get k = Value "some_data" works ok. Also, you can make the type of get polymorphic (accepting different data types) by writing get :: a -> Value a get k = Value k where whatever the type of a is, get will return Value of that type (Value a). This works ok because the type of your data type is also polymorphic: data Value b = Value b deriving (Show) i.e., b can be something of any type. Hope this helps. Cheers, Pedro On Dec 7, 2012, at 2:02 PM, Alexander _ wrote:
Hello,
I have function with following signature:
get :: a -> Value b
I don't know concrete type of b. It can be Value Int or Value String or something else. How can i to write function only for testing? I need something like this:
get :: a -> Value b get k = (Value "some_data"')
but i got error:
Couldn't match expected type `b' with actual type `[Char]' `b' is a rigid type variable bound by the type signature for get :: a -> Value b at Test.hs:39:8 In the first argument of `Value', namely `"some_data"' In the expression: (Value "some_data") In an equation for `get': get k = (Value "some_data") Failed, modules loaded: none.
Value data declaration:
data Value b = Value b deriving (Show)
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