CODEWITHSUNDEEP
haskell
Michael Jackson
Michael Jackson

Reputation: 13

Seemingly odd output for Haskell function

I have a Haskell function:

sumSquareOrSquareSum' :: (Num a, Ord a) => a -> a -> a
sumSquareOrSquareSum' x y = (\sumSquare squareSum ->
                              if sumSquare > squareSum
                              then sumSquare
                              else squareSum) (x^2 + y^2) (x+y)^2

If I run it as sumSquareOrSquareSum' 1 2 I get the answer 25 instead of the 9 I would expect.

If I make both branches return squareSum I get 9, if both return sumSquare I get 25. If I make the GT a LT I get 9, so it appears to be the first term that's evaluating weirdly, or at least in a manner that I don't understand.

I can fix the problem by putting parentheses around the (x+y)^2 term making the final line:

else squareSum) (x^2 + y^2) ((x+y)^2)

So, I know how to fix it, but I'm not at all clear as to why there is a problem in the first place. Any insight would be appreciated.

Upvotes: 1

Views: 53

Answers (2)

Franky
Franky

Reputation: 2411

Function application binds more tighly than operators, so

f x^2

is evaluated as

(f x)^2

instead of the

f (x^2)

you expect. This still applies to functions with two parameters, so

max a b^2

is actually

(max a b)^2

Flip the parameters like max b^2 a and see the error message as haskell is trying to interpret that as (max b)^(2 a).

Upvotes: 2

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477462

It is interpreted as:

sumSquareOrSquareSum' :: (Num a, Ord a) => a -> a -> a
sumSquareOrSquareSum' x y = ((\sumSquare squareSum ->
                              if sumSquare > squareSum
                              then sumSquare
                              else squareSum) (x^2 + y^2) (x+y))^2

So you square the result of the maximum of the sum of squares, and the sum. So you passed (x^2 + y^2) as first parameter (1*1+2*2 is 5) , and the sum (1+2 is 3) as second parameter. This is thus returns 5, and then we square it, so 25.

You thus returned the square of the maximum of the sum of squares and the sum.

Upvotes: 1

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