haskell - Restricting monad type in data constructor -

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i'm looking create small monadic interpreter in haskell , ran problem ghc complaining me kinds don't match up.

specifically, here declaration data type value:

data value = wrong            | num int            | fun (value -> monad value)

and when compile this, ghc gives following error message:

the first argument of ‘monad’ should have kind ‘* -> *’,   ‘value’ has kind ‘*’ in type ‘value -> (monad value)’ in definition of data constructor ‘fun’ in data declaration ‘value’

but monad value of kind *. don't see problem here?

could kindly point me bug i'm not seeing? lot!

p.s. interpreter based on 1 introduced in wadler's paper the essence of functional programming.

essentially, monad expects type constructor takes 1 argument. value type constructor takes no arguments. so, value must have kind * -> * (meaning takes 1 argument) instead of * (meaning takes no arguments).

also, if value did take 1 argument (allowing type check), monad value of kind constraint. because monad of kind (* -> *) -> constraint , in monad value being applied of kind * -> *.


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