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Idiomatic Haskell

What I'm picking up is that idiomatic Haskell is exemplified through the use of its built-in functions. Consider looking for an existing function and combining it as needed with other functions rather than trying to use operators (which you'll need to do, yes, but focus on functions first; it is a functional language after all).

Though I'm reminded any Haskell is good Haskell; let the compiler do the work of making things efficient

-- this
last xs = head (reverse xs)
-- not this
last xs = xs !! (length xs - 1)

Tip Some is from What to avoid in the Prelude, due to historical changes.

  • Prefer fmap over map.
  • Avoid String.
  • Use Foldable and Traversable instead of the Control.Monad, and Data.List versions of traversals.
  • Avoid [using or creating] partial functions like head and read or use their total variants.
  • Avoid exceptions, use ExceptT or Either instead.
  • Avoid boolean blind functions.
  • Prefer unwords to ++ since it works on Text as well as String, whereas ++ only works on lists.

Or, just pull the Prelude in explicitly and use what you need:

import qualified Prelude as P
-- or, for example, to hide what might conflict.
import Prelude hiding (length)

More Complex Idioms​

-- this:
(+) <$> [2,3,4] <*> pure 4.

-- is more idiomatic than this:
pure (+) <*> [2,3,4] <*> pure 4