# Difference between revisions of "Avoiding IO"

(randomIO) |
(ST monad) |
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evalState (randomDist distInv) (mkStdGen an_arbitrary_seed) | evalState (randomDist distInv) (mkStdGen an_arbitrary_seed) | ||

</haskell> | </haskell> | ||

− | |||

== ST monad == | == ST monad == | ||

− | STRef | + | In some cases a state monad is simply not efficient enough. |

+ | Say the state is an array and the update operations are modification of single array elements. | ||

+ | For this kind of application the [[Monad/ST|State Thread monad]] <hask>ST</hask> was invented. | ||

+ | It provides <hask>STRef</hask> as replacement for <hask>IORef</hask>, | ||

+ | <hask>STArray</hask> as replacement for <hask>IOArray</hask>, | ||

+ | <hask>STUArray</hask> as replacement for <hask>IOUArray</hask>, | ||

+ | and you can define new operations in ST, but then you need to resort to unsafe operations. | ||

+ | You can escape from ST to non-monadic code in a safe, and in many cases efficient, way. | ||

== Custom type class == | == Custom type class == | ||

Line 83: | Line 89: | ||

example getText | example getText | ||

+ | |||

+ | == Last resort == | ||

+ | |||

+ | The method of last resort is <hask>unsafePerformIO</hask>. | ||

+ | When you apply it, think about how to reduce its use | ||

+ | and how you can encapsulate it in a library with a well chosen interface. | ||

+ | You may define new operations in the <hask>ST</hask> monad using <hask>unsafeIOToST</hask>. | ||

[[Category:Monad]] | [[Category:Monad]] | ||

[[Category:Idioms]] | [[Category:Idioms]] | ||

− | [[Category:Style]] | + | [[Category:Style]] |

## Revision as of 16:46, 25 December 2008

Haskell requires an explicit type for operations involving input and output.
This way it makes a problem explicit, that exists in every language:
Input and output functions can have so many effects, that the type signature says more or less that almost everything must be expected.
It is hard to test them, because they can in principle depend on every state of the real world.
Thus in order to maintain modularity you should avoid IO whereever possible.
It is too tempting to get rid of IO by `unsafePerformIO`

,
but we want to present some clean techniques to avoid IO.

## Lazy construction

You can avoid a series of output functions by constructing a complex data structure with non-IO code and output it with one output function.

Instead of

```
-- import Control.Monad (replicateM_)
replicateM_ 10 (putStr "foo")
```

you can also create the complete string and output it with one call of `putStr`

:

```
putStr (concat $ replicate 10 "foo")
```

Similarly,

```
do
h <- openFile "foo" WriteMode
replicateM_ 10 (hPutStr h "bar")
hClose h
```

can be shortened to

```
writeFile "foo" (concat $ replicate 10 "bar")
```

which also ensures proper closing of the handle `h`

in case of failure.

Since you have now an expression for the complete result as string,
you have a simple object that can be re-used in other contexts.
E.g. you can also easily compute the length of the written string using `length`

without bothering the file system, again.

## State monad

If you want to maintain a running state, it is tempting to use `IORef`

.
But this is not necessary, since there is the comfortable `State`

monad and its transformer counterpart.

Another example is random number generation. In cases where no real random numbers are required, but only arbitrary numbers, you do not need access to the outside world. You can simply use a pseudo random number generator with an explicit state. This state can be hidden in a State monad.

Example: A function which computes a random value
with respect to a custom distribution
(`distInv`

is the inverse of the distribution function)
can be defined via IO

```
randomDist :: (Random a, Num a) => (a -> a) -> IO a
randomDist distInv = liftM distInv (randomRIO (0,1))
```

but there is no need to do so. You don't need the state of the whole world just for remembering the state of a random number generator. What about

```
randomDist :: (RandomGen g, Random a, Num a) => (a -> a) -> State g a
randomDist distInv = liftM distInv (State (randomR (0,1)))
```

? You can get actual values by running the `State`

as follows:

```
evalState (randomDist distInv) (mkStdGen an_arbitrary_seed)
```

## ST monad

In some cases a state monad is simply not efficient enough.
Say the state is an array and the update operations are modification of single array elements.
For this kind of application the State Thread monad `ST`

was invented.
It provides `STRef`

as replacement for `IORef`

,
`STArray`

as replacement for `IOArray`

,
`STUArray`

as replacement for `IOUArray`

,
and you can define new operations in ST, but then you need to resort to unsafe operations.
You can escape from ST to non-monadic code in a safe, and in many cases efficient, way.

## Custom type class

example getText

## Last resort

The method of last resort is `unsafePerformIO`

.
When you apply it, think about how to reduce its use
and how you can encapsulate it in a library with a well chosen interface.
You may define new operations in the `ST`

monad using `unsafeIOToST`

.