Do notation considered harmful

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Revision as of 06:59, 5 November 2007 by Lemming (talk | contribs) (didactic problems)

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Haskell's do notation is popular and ubiquitous. However we shall not ignore that there are several problems. Here we like to shed some light on aspects you may not have thought about, so far.


The do notation hides functional details. This is wanted in order to simplify writing imperative style code fragments. The downsides are

  • that, since do notation is used almost everywhere, where IO takes place, newcomers quickly believe that the do notation is necessary for doing IO,
  • and that newcomers think, that IO is somehow special and non-functional, in contrast to the advertisement for Haskell being purely functional.

These misunderstandings let people write clumsy code like

do putStrLn "text"

instead of

putStrLn "text"


do text <- getLine
   return text

instead of



  text <- readFile "foo"
  writeFile "bar" text

instead of

readFile "foo" >>= writeFile "bar"


Library design

See also