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Do notation considered harmful

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Contents

1 Criticism

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.

1.1 Didactics

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"

or

do text <- getLine
   return text

instead of

getLine

or

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

instead of

readFile "foo" >>= writeFile "bar"

.

1.2 Library design

2 See also