haskell — not understanding why this associated ty

Question

Consider the following code,

data MyBaseExpr α where
    ConstE :: Show α => α -> MyBaseExpr α

class Monad                 

            

标签： haskell typeclass associated-types

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Answer 1

ExprTyp is not (necessarily) an injective type function. This means that being handed something of type ExprType m doesn't nail down m -- there may also be a different n such that ExprType n = ExprType m. This makes the type of expTypArg a bit tricky: it uses return-type polymorphism in the same way, e.g., read does, so you'll need to give extra type annotations to its result in the same situations you have to do with read.