Unsure of how to get the right evaluation order

Posted by Matt Fenwick on Stack Overflow See other posts from Stack Overflow or by Matt Fenwick
Published on 2012-10-24T16:47:20Z Indexed on 2012/10/24 17:00 UTC
Read the original article Hit count: 294

Filed under:

haskell

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ghci

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order-of-evaluation

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strictness

I'm not sure what the difference between these two pieces of code is (with respect to x), but the first one completes:

$ foldr (\x y -> if x == 4 then x else x + y) 0 [1,2 .. ]
10

and the second one doesn't (at least in GHCi):

$ foldr (\x (y, n) -> if x == 4 then (x, n) else (x + y, n + 1)) (0, 0) [1,2 .. ]
.......

What am I doing wrong that prevents the second example from completing when it hits x == 4, as in the first one?

I've tried adding bang-patterns to both the x and to the x == 4 (inside a let) but neither seems to make a difference.

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