# Difference between revisions of "Avoiding IO"

(ST monad) |
(custom monad class) |
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You can escape from ST to non-monadic code in a safe, and in many cases efficient, way. |
You can escape from ST to non-monadic code in a safe, and in many cases efficient, way. |
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− | == Custom type class == |
+ | == Custom monad type class == |

− | example getText |
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+ | If you only use a small set of IO operations in otherwise non-IO code |
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+ | you may define a custom monad type class which implements just these functions. |
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+ | You can then implement these functions based on IO for the application and without IO for the test suite. |
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+ | As an example consider the function |
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+ | <haskell> |
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+ | localeTextIO :: String -> IO String |
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+ | </haskell> |
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+ | which converts an English phrase to the currently configured user language of the system. |
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+ | You can abstract the <hask>IO</hask> away using |
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+ | <haskell> |
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+ | class Locale m where |
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+ | localeText :: String -> m String |
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+ | |||

+ | instance Locale IO where |
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+ | localeText = localeTextIO |
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+ | |||

+ | instance Locale Identity where |
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+ | localeText = Identity |
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+ | </haskell> |
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+ | where the first instance can be used for the application and the second one for "dry" tests. |
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+ | For more sophisticated tests, you may load a dictionary into a <hask>Map</hask> and use this for translation. |
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+ | <haskell> |
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+ | newtype Interpreter a = Interpreter (Reader (Map String String) a) |
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+ | |||

+ | instance Locale Interpreter where |
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+ | localeText text = Interpreter $ fmap (Map.findWithDefault text text) ask |
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+ | </haskell> |
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== Last resort == |
== Last resort == |

## Revision as of 16:59, 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 monad type class

If you only use a small set of IO operations in otherwise non-IO code you may define a custom monad type class which implements just these functions. You can then implement these functions based on IO for the application and without IO for the test suite.

As an example consider the function

```
localeTextIO :: String -> IO String
```

which converts an English phrase to the currently configured user language of the system.
You can abstract the `IO`

away using

```
class Locale m where
localeText :: String -> m String
instance Locale IO where
localeText = localeTextIO
instance Locale Identity where
localeText = Identity
```

where the first instance can be used for the application and the second one for "dry" tests.
For more sophisticated tests, you may load a dictionary into a `Map`

and use this for translation.

```
newtype Interpreter a = Interpreter (Reader (Map String String) a)
instance Locale Interpreter where
localeText text = Interpreter $ fmap (Map.findWithDefault text text) ask
```

## 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`

.