Haskell: Couldn't match expected type ``Int' with actual type ``Integer'
Date: 2012-03-13 19:42:42

One of the most frequent compilation error messages that I've been getting while working through the Project Euler problems in Haskell is the following:

[source,text]
----

Couldn't match expected type `Int' with actual type `Integer'
----

In http://projecteuler.net/problem=11[problem 11], for example, I define the grid of numbers like so:

[source,haskell]
----

grid = [[08,02,22,97,38,15,00,40,00,75,04,05,07,78,52,12,50,77,91,08],
        [49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48,04,56,62,00],
        [81,49,31,73,55,79,14,29,93,71,40,67,53,88,30,03,49,13,36,65],
        [52,70,95,23,04,60,11,42,69,24,68,56,01,32,56,71,37,02,36,91],
        [22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80],
        [24,47,32,60,99,03,45,02,44,75,33,53,78,36,84,20,35,17,12,50],
        [32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70],
        [67,26,20,68,02,62,12,20,95,63,94,39,63,08,40,91,66,49,94,21],
        [24,55,58,05,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72],
        [21,36,23,09,75,00,76,44,20,45,35,14,00,61,33,97,34,31,33,95],
        [78,17,53,28,22,75,31,67,15,94,03,80,04,62,16,14,09,53,56,92],
        [16,39,05,42,96,35,31,47,55,58,88,24,00,17,54,24,36,29,85,57],
        [86,56,00,48,35,71,89,07,05,44,44,37,44,60,21,58,51,54,17,58],
        [19,80,81,68,05,94,47,69,28,73,92,13,86,52,17,77,04,89,55,40],
        [04,52,08,83,97,35,99,16,07,97,57,32,16,26,26,79,33,27,98,66],
        [88,36,68,87,57,62,20,72,03,46,33,67,46,55,12,32,63,93,53,69],
        [04,42,16,73,38,25,39,11,24,94,72,18,08,46,29,32,40,62,76,36],
        [20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74,04,36,16],
        [20,73,35,29,78,31,90,01,74,31,49,71,48,86,81,16,23,57,05,54],
        [01,70,54,71,83,51,54,69,16,92,33,48,61,43,52,01,89,19,67,48]]
----

Which has the following type:

[source,haskell]
----

> :t grid
grid :: [[Integer]]
----

The problem is that I wrote some functions to pass the grid to which expect it to be an array of array of +++<cite>+++Ints+++</cite>+++. For example the following function helps find the diagonals left to right from our current position:

[source,haskell]
----

-- implementation of findValue and hasItem isn't relevant here...

diagonalAt :: Int -> Int -> [[Int]] -> [Int]
diagonalAt x y grid = [findValue x y grid z | z <- [0..(length grid)],
						hasItem grid (x + z),
						hasItem (grid !! (x + z)) (y + z)]
----

[source,text]
----

> diagonalAt 0 0 grid
    Couldn't match expected type `Int' with actual type `Integer'
    Expected type: [[Int]]
      Actual type: [[Integer]]
    In the third argument of `diagonalAt', namely `x'
    In the expression: diagonalAt 0 0 x
----

But if we give a type definition instead of allowing it to be inferred...

[source,haskell]
----

grid :: [[Int]]
-- definition of grid as before
----

...then it will be of the type we want:
[source,text]
----

> :t grid
grid :: [[Int]]
----

Alternatively we can make use of the +++<cite>+++http://www.haskell.org/haskellwiki/Converting_numbers[fromIntegral]+++</cite>+++ function to convert from +++<cite>+++Integer+++</cite>+++ to +++<cite>+++Int+++</cite>+++ which in this situation leads to the following code:

[source,haskell]
----

> diagonalAt 0 0 (map (map fromIntegral) grid)
[8,49,31,23,51,3,67,20,97,45,3,24,44,52,26,32,40,4,5,48]
----