CODEWITHSUNDEEP
haskell
Alin Ciocan
Alin Ciocan

Reputation: 3068

Haskell: How to match a function pattern with recursively types?

I have this type recursively:

data Expr = Add Expr Expr | Num Int

add (Add (Num a) (Num b)) = a + b

For add Add ((Num 1) (Num 2)) works, but how can I do for all recursively types, like:

add Add( (Add (Num 1) (Num 2)) (Num 3) ) ?

Upvotes: 3

Views: 570

Answers (1)

valderman
valderman

Reputation: 8755

Since you have no way of knowing just how deep a value of your Expr type nests, you have to write a recursive function to evaluate it rather than trying to take on the entire expression at once.

For your Expr type, it would look something like this:

eval :: Expr -> Int
eval (Add a b) = eval a + eval b
eval (Num a)   = a

In general, when you have a recursive type like this, you're going to need a recursive function to do anything really interesting with it.

Upvotes: 7

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