Headline

Constrain continuation of computation based on Boolean expression

Illustration

The notion of guard is pretty general as it occurs in different ways in programming languages which is why we do not attempt a comprehensive description here. Instead, we explain guards in Haskell; they are helpful here for adding more control on pattern matching.

Guards in Haskell

Consider the following function for finding the minimum in a list:

findmin (x:xs) = findmin' x xs
  where
    findmin' m [] = m
    findmin' m (y:ys) =
      if m < y
        then findmin' m ys
        else findmin' y ys

Arguably, it may be preferable to decompose the second case for non-empty list into two cases: one for each branch of the if-then-else. We can guard the equations for the two cases accordingly.

findmin (x:xs) = findmin' x xs
  where
    findmin' m [] = m
    findmin' m (y:ys) | m < y = findmin' m ys
    findmin' m (y:ys) | m >= y = findmin' y ys

Arguably, the guard for the last equation is not needed because the equation would only be tried in a case where the condition must evidently hold. So we may want to simplify as follows.

findmin (x:xs) = findmin' x xs
  where
    findmin' m [] = m
    findmin' m (y:ys) | m < y = findmin' m ys
    findmin' m (y:ys) = findmin' y ys

Arguably, we do not want to use guards at all, as we may be even more concise (without guards) as follows.

findmin (x:xs) = findmin' x xs
  where
    findmin' m [] = m
    findmin' m (y:ys) = findmin' (min m y) ys

The guards of Haskell provide yet additional expressiveness that we omit here for brevity.


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