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haskellstack-overflow

Why does MC Haskell function result in stack overflow

I am working my way through the Haskell book and am on chapter 8. While doing the exercises I noticed something I didn't understand.

Why does this result in stack overflow 

mc x | x>100 = x-10
     | otherwise = mc $ mc x+11

but this doesn't

mc x | x>100 = x-10
     | otherwise = mc $ mc (x+11)

I think it has something to do with x+11 not being evaluated in the first example but aren't expressions like that always evaluated

for example 

Prelude> id 43+94
137
Solution

  • The first expression

    mc $ mc x+11

    is interpreted as

    mc ((mc x) + 11)

    since function application takes precedence over operators.

    The second expression

    mc $ mc (x+11)

    is interpreted as:

    mc (mc (x+11))

    The first indeed will never get evaluated, since if you write:

    mc x | x > 100 = x-10
     | otherwise = mc ((mc x) + 11)

    then you define mc x in terms of mc x. Unless that mc x in that expression is not evaluated, you thus will call mc x, when calculating mc x, and thus it will keep making calls.