Data definition error haskell

Question

I'm trying to define a polymorphic list of tuples without using any imports. i have data ListT a = [(a,String)] deriving (Eq,Ord,Read,Show) but it keeps giving the error "cannot parse data constructor in a data/newtype declaration: [(a,String)]"

What does this error mean, and how do I prevent it. Thanks

To avoid confusion you might want to choose a different name - ListT is used in the Haskell community for the monad transformer for the List type. — ErikR
– ErikR, Commented Oct 20, 2015 at 20:29

Nikita Volkov · Accepted Answer · 2015-10-20 20:25:32Z

7

It means that you forgot to provide the name for the value constructor. You need to change your code to the following:

data ListT a = ListT [(a,String)]

BTW, it is not a proper implementation of List Transformer.

answered Oct 20, 2015 at 20:25

Nikita Volkov

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