Haskell - defining a function with guards inside a 'where'

haskell, parse-error, where-clause

Solution

lowestDivisor :: Integer -> Integer
lowestDivisor n = lowestDivisorHelper 2 n where 
  lowestDivisorHelper m n
        | (m `divides` n) = m  -- these should belong to lowestDivisorHelper
        | otherwise = lowestDivisorHelper (m+1) n

You need to start a new statement with your helper function for the guards to be sufficiently indented by comparison. (And you also forgot an argument, `n`.) This would also work:

lowestDivisor :: Integer -> Integer
lowestDivisor n = lowestDivisorHelper 2 n 
    where 
  lowestDivisorHelper m n
        | (m `divides` n) = m  -- these should belong to lowestDivisorHelper
        | otherwise = lowestDivisorHelper (m+1) n

but this doesn't:

lowestDivisor :: Integer -> Integer
lowestDivisor n = lowestDivisorHelper 2 n 
  where lowestDivisorHelper m n
        | (m `divides` n) = m  -- these should belong to lowestDivisorHelper
        | otherwise = lowestDivisorHelper (m+1) n

The key point is that the `|` has to be further to the right than the function name.

In general, starting a new line continues the previous one as long as it is further to the right. The guards have to continue on from the function name.

Problem

I'm just starting out at teaching myself Haskell. This code is supposed to do prime factorisation: ``` divides :: Integer -> Integer -> Bool divides small big = (big `mod` small == 0) lowestDivisor :: Integer -> Integer lowestDivisor n = lowestDivisorHelper 2 n where lowestDivisorHelper m n | (m `divides` n) = m -- these should belong to lowestDivisorHelper | otherwise = lowestDivisorHelper (m+1) n primeFactors :: Integer -> [Integer] primeFactors 1 = [] primeFactors n | n < 1 = error "Must be positive" | otherwise = let m = lowestDivisor n in m:primeFactors (n/m) ``` I get a parse error on the commented line. I think my problem might be that `lowestDivisorHelper` has guards, but the compiler doesn't know whether the guards belong to `lowestDivisorHelper` or `lowestDivisor`. How do I get around this? I should add that I didn't want to define the helper function at the top level in order to hide the implementation detail. Importing the file shouldn't take the helper function with it.

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