CODEWITHSUNDEEP
haskellpattern-matchingalgebraic-data-types
barskyn
barskyn

Reputation: 383

Pattern matching Over A Nested Data Constructor

For the following code I keep on getting a compiler error because I am writing (List _) and it is trying to pattern match over the list. How can I fix tihs?

data List a = Cons a (List a) | Empty

instance Monad List where
    Empty >>= _ = Empty 
    Cons x (List _) >>= f = f x

Upvotes: 0

Views: 221

Answers (1)

Jon Purdy
Jon Purdy

Reputation: 54979

data List a = Cons a (List a) | Empty

This defines a type constructor List with two data constructors Cons and Empty. Cons has two fields, one of type a, and one of type List a—which may, in turn, be either Empty or a Cons, and so on.

So when you write:

Cons x (List _) >>= f = f x

The compiler is looking for a data constructor called List to pattern-match on, and there isn’t one—hence the error. If you just want to ignore the field, write:

Cons x _ >>= f = f x

Now of course this isn’t a proper Monad instance for List—it’s more like the one for Maybe—but I’ll leave you to work through that on your own.

Upvotes: 7

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