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Top-level vs. local recursion

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Compare the following two implementations of map. The first one uses top-level recursion

map :: (a -> b) -> [a] -> [b]
map _ []     = []
map f (x:xs) = f x : map f xs

whereas the second one uses local recursion

map :: (a -> b) -> [a] -> [b]
map f =
   let go []     = []
       go (x:xs) = f x : go xs
   in  go

Although the first version is shorter, there some reasons to prefer the second version for stylistic reasons:

  • It clearly shows that the 'f' is not "altered" in the recursion.
  • You cannot accidentally 'jump' into the wrong loop. I often have multiple implementations of the same function which differ in laziness or performance. They only differ slightly in name and it happened too often that after copy&paste the recursive call went to a different (and thus wrong) function.
  • The local loop is probably also more efficient in execution, because the compiler does not need to move the
    around. However, if this is actually more efficient then the compiler should do such a transformation itself.
Btw. the best implementation is probably
foldr (\x acc -> f x : acc) []

which is both short and allows deforestation.

See also