Do notation considered harmful

From HaskellWiki

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

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

• Paul Hudak in Haskell-Cafe: A regressive view of support for imperative programming in Haskell
• Data.Syntaxfree on Wordpress: Do-notation considered harmful

Category: Syntax