# Do notation considered harmful

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## Revision as of 18:10, 14 January 2010

## 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

TheThis is wanted in order to simplify writing imperative style code fragments. The downsides are

- that, since notation is used almost everywhere, wheredotakes place, newcomers quickly believe that theIOnotation is necessary for doingdo,IO
- that newcomers think, that is somehow special and non-functional, in contrast to the advertisement for Haskell being purely functional,IO
- and that newcomers think, that the order of statements determines the order of execution.

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"

.

The order of statements is also not the criterion for the evaluation order. Also here only the data dependencies count. See for instance

do x <- Just (3+5) y <- Just (5*7) return (x-y)

Or consider

do x <- Just (3+5) y <- Nothing return (x-y)

### 1.2 Library design

Unfortunately, theSee for instance the Binary package.

It contains theEven more unfortunate, the applicative functors were introduced to Haskell's standard libraries only after monads and arrows,

thus many types are instances ofThere is no special syntax for applicative functors because it is hardly necessary. You just write

data Header = Header Char Int Bool readHeader :: Get Header readHeader = liftA3 Header get get get

or

readHeader = Header <$> get <*> get <*> get

Consider a generator of unique identifiers.

First you might think of arun :: State Int a -> a run m = evalState m 0 newId :: State Int Int newId = do n <- get modify succ return n example :: (Int -> Int -> a) -> a example f = run $ do x <- newId y <- newId return (f x y)

If you are confident, that you will not need the counter state at the end and
that you will not combine blocks of code using the counter
(where the second block needs the state at the end of the first block),
you can enforce a more strict scheme of usage.

Alternatively you can view it as Continuation monad.

newtype T a = T (Int -> a) run :: T a -> a run (T f) = f 0 newId :: (Int -> T a) -> T a newId f = T $ \i -> case f i of T g -> g (succ i) example :: (Int -> Int -> T a) -> a example f = run $ newId $ \a -> newId $ \b -> f a b

### 1.3 Safety

*This page addresses an aspect of Haskell style, which is to some extent a matter of taste. Just pick what you find appropriate for you and ignore the rest.*

The silent neglect of return values of functions. In an imperative language it is common to return an error code and provide the real work by side effects. In Haskell this cannot happen, because functions have no side effects. If you ignore the result of a Haskell function the function will even not be evaluated.

The situation is different forYou can write

do getLine putStrLn "text"

The same applies to

do System.Cmd.system "echo foo >bar"

Is this behaviour wanted?

In safety oriented languages there are possibilities to explicitly ignore return values
(e.g. `EVAL`

in Modula-3).
Haskell does not need this, because you can already write

do _ <- System.Cmd.system "echo foo >bar" return ()

(>>) :: m () -> m a -> m a

New developments:

- GHC since version 6.12 emits a warning when you silently ignore a return value
- There is a new function called that makes ignoring of return values explicit: GHC ticket 3292void

## 2 Happy with less sugar

### 2.1 Additional combinators

Using the infix combinators for writing functions simplifies the addition of new combinators. Consider for instance a monad for random distributions.

This monad cannot be an instance ofHowever we would like to write the following:

do f <- family guard (existsBoy f) return f

we can rewrite this easily:

`family >>=? existsBoy`

but it seems that we have to live with that problem.

### 2.2 Alternative combinators

If you are used to write monadic function using infix combinatorsyou can easily switch to a different set of combinators.

This is useful when there is a monadic structure that does not fit into the currentwhere the monadic result type cannot be constrained. This is e.g. useful for the Set data type, where the element type must have a total order.

## 3 Useful applications

It shall be mentioned that theE.g. in

getRight :: Either a b -> Maybe b getRight y = do Right x <- y return x

Compare

mdo x <- f x y z y <- g x y z z <- h x y z return (x+y+z)

and

mfix (\ ~( ~(x,y,z), _) -> do x <- f x y z y <- g x y z z <- h x y z return ((x,y,z),x+y+z))

## 4 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
- Things to avoid#do notation