CODEWITHSUNDEEP
haskell
wtl
wtl

Reputation: 3

How do I declare new type with type constraints in Haskell?

I’m tring to declare a new type Value as below:

data Value m = Wrong | Num Int | Fun (Value -> m Value)

And GHCI complains:

<interactive>:160:39:
    Expecting one more argument to ‘Value’
    Expected a type, but ‘Value’ has kind ‘k0 -> *’
    In the type ‘Value -> m Value’
    In the definition of data constructor ‘Fun’
    In the data declaration for ‘Value’

PS: I'm trying to implement code of The essence of functional programming

Upvotes: 0

Views: 165

Answers (1)

sepp2k
sepp2k

Reputation: 370455

Since Value takes a type parameter, you need to supply that parameter any time you use Value. That is, you should refer to it as Value m, not just Value. So your type definition should be:

data Value m = Wrong | Num Int | Fun ((Value m) -> m (Value m))

Upvotes: 4

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