问题
I am a new learner of Haskell, my code is as follows:
data Num a=>Units a = Units a (SymbolicManip a )
deriving (Eq)
I am not sure how to fix it?
Anyone can help me?
回答1:
Typeclass contexts in datatypes are now regarded as a not so useful feature. The problem is that the following does not compile:
foo :: Units a -> a
foo (Units x _) = x+x
This intuitively should compile, since the Units a argument can only be constructed for a type a satisfying Num a. So, on destruction (pattern matching) one should be able to access the Num a instance. However this is not the case, and a Num a must be counterintuitively provided on destruction as well:
foo :: Num a => Units a -> a
foo (Units x _) = x+x
The standard suggestion is therefore to remove the constraint Num a from the Units a datatype declaration, and add it instead to every function involving Units a.
Another option is to enable GADTs and change the datatype to:
data Units a where
Units :: Num a => a -> SymbolicManip a -> Units a
This does the "right" thing: a Num a instance is required to construct a value, and is instead provided on destruction. In this way, the first foo declaration above will be well-typed.
I almost forgot the "quick & dirty" option, which is to enable the obsolescent datatype context feature: this is done by adding at the beginning of your file the line
{-# LANGUAGE DatatypeContexts #-}
Still, I would rather modify the code than to enable this language extension.
来源：https://stackoverflow.com/questions/22622399/how-to-fix-illegal-datatype-context-use-xdatatypecontexts