Fernando Rodriguez wrote:
Hi,
I'm trying to write a function that finds out the week day using the Zeller congruence (http://en.wikipedia.org/wiki/Zeller's_congruence).
However, ghc complains about: parse error on input `=' at the "if m <= 2 then " line. I believe this is some sort of layout error. Can someone point out what I am doing wrong?
Here's the code:
data DiaSemana = Lunes | Martes | Miercoles | Jueves | Viernes | Sabado | Domingo deriving (Show, Eq, Ord, Enum) diaDeSemana d m a = toEnum num :: DiaSemana where num = zeller x y z zeller x y z = (700 + (26 * x - 2) `div` 10 + d + y + y `div` 4 + z `div` 4 - 2 * z) `mod` 7 if m <= 2 then x = m + 10 y = (a - 1) `mod` 100 z = (a-1) 'div' 100 else x = m - 2 y = a `mod` 100 z = a `div` 100
That looks like it's been mangled a bit by your mail program or mine, but anyway: This is not a layout problem. The problem here is simply that you can't put "ifs" around chunks of definitions. "if" is an expression level concept, not a definition-level one. You could try: diaDeSemana d m a = toEnum num :: DiaSemana where num = zeller x y z zeller x y z = (700 + (26 * x - 2) `div` 10 + d + y + y `div` 4 + z `div` 4 - 2 * z) `mod` 7 x | m <= 2 = m + 10 | otherwise = m - 2 y | m <= 2 = (a - 1) `mod` 100 | otherwise = a `mod` 100 z | m <= 2 = (a-1) 'div' 100 | otherwise = a `div` 100 or, to avoid the ugly duplication of the m <= 2 constraint, we can use an if in an expresson like this: diaDeSemana d m a = toEnum num :: DiaSemana where num = zeller x y z zeller x y z = (700 + (26 * x - 2) `div` 10 + d + y + y `div` 4 + z `div` 4 - 2 * z) `mod` 7 (x,y,z) = if m <=2 then (m+10,(a-1)`mod`100,(a-1)`div`100) else (m-2,a`mod`100,a`div`100) ... or, replacing the if with guards again ... diaDeSemana d m a = toEnum num :: DiaSemana where num = zeller x y z zeller x y z = (700 + (26 * x - 2) `div` 10 + d + y + y `div` 4 + z `div` 4 - 2 * z) `mod` 7 (x,y,z) | m <=2 = (m+10,(a-1)`mod`100,(a-1)`div`100) | otherwise = (m-2 ,a `mod`100,a `div`100) Jules