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Do notation considered harmful

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(counter with Reader monad, but without monad)
(modula3.org)
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In safety oriented languages there are possibilities to explicitly ignore return values
 
In safety oriented languages there are possibilities to explicitly ignore return values
(e.g. <code>EVAL</code> in [http://www.m3.org/ Modula-3]).
+
(e.g. <code>EVAL</code> in [http://www.modula3.org/ Modula-3]).
 
Haskell does not need this, because you can already write
 
Haskell does not need this, because you can already write
 
<haskell>
 
<haskell>

Revision as of 06:39, 29 November 2007

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

Unfortunately, the
do
notation is so popular that people write more things with monads than necessary.

See for instance the Binary package.

It contains the
Put
monad, which has in principle nothing to do with a monad.

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

thus many types are instances of
Monad
and
Arrow
classes, but not as much are instances of
Applicative
.

There 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
Not using monads and thus
do
notation can have advantages.

Consider a generator of unique identifiers.

First you might think of a
State
monad which increments a counter each time an identifier is requested.
run :: 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.

The following is like a
Reader
monad, where we call
local
on an incremented counter for each generated identifier.
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
This way users cannot accidentally place a
return
somewhere in a
do
block where it has no effect.


1.3 Safety

With
do
notation we have kept alive a dark side of the C programming language:

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 for
IO
: While processing the
IO
you might still ignore the contained return value.

You can write

do getLine
   putStrLn "text"
and thus silently ignore the result of
getLine
.

The same applies to

do System.cmd.system "echo foo >bar"
where you ignore the
ExitCode
.

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 ()
Writing
 _ <-
should always make you cautious whether ignoring the result is the right thing to do. The possibility for silently ignoring monadic return values is not entirely the fault of the
do
notation. It would suffice to restrict the type of the
(>>)
combinator to
(>>) :: m () -> m a -> m a
This way, you can omit
 _ <-
only if the monadic return value has type
()
.


2 Useful applications

It shall be mentioned that the
do
sometimes takes the burden from you to write boring things.

E.g. in

getRight :: Either a b -> Maybe b
getRight y =
   do Right x <- y
      return x
a
case
on
y
is included, which calls
fail
if
y
is not a
Right
(i.e.
Left
), and thus returns
Nothing
in this case. Also the
mdo
notation proves useful, since it maintains a set of variables for you in a safe manner.

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))


3 See also