# Difference between revisions of "Type signatures as good style"

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+ | == Question == |
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+ | |||

+ | Since Haskell type checkers can automatically [[Determining_the_type_of_an_expression|derive types of expressions]] |
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+ | why shall I put explicit type signatures in my programs? |
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+ | |||

+ | == Answer == |
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+ | |||

+ | Using explicit [[type signature]]s is good style and [[GHC]] with option <code>-Wall</code> warns about missing signatures. |
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+ | Signatures are a good documentation and not all Haskell program readers have a type inference algorithm built-in. |
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+ | There are also some cases where the infered signature is too general for your purposes. |
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+ | E.g. the infered (most general) type for <hask>asTypeOf</hask> is <hask>a -> b -> a</hask>, |
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+ | but the purpose of <hask>asTypeOf</hask> is to unify the types of both operands. |
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+ | The more special signature <hask>a -> a -> a</hask> is what you want and it cannot be infered automatically. |
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+ | Another example: |
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+ | <haskell> |
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+ | emptyString :: ShowS |
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+ | emptyString = id |
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+ | </haskell> |
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+ | Where <hask>ShowS</hask> is <hask>String -> String</hask> rather than <hask>a -> a</hask>. |
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+ | |||

+ | Even more, for some type extensions the automatic inference fails, |
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+ | e.g. the higher-order types used by <hask>Control.Monad.ST.runST</hask> |
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+ | <haskell> |
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+ | runST :: (forall s . ST s a) -> a |
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+ | </haskell> |
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+ | cannot be inferred in general, because the problem is undecidable. In GHC, they are enabled with the language pragma <code>RankNTypes</code>. |
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+ | |||

+ | == How to add a bunch of signatures? == |
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+ | |||

+ | Ok, this convinced me. How can I add all the signatures I did not write so far? |
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+ | |||

+ | : You can start [[GHC|GHCi]] or [[Hugs]] and use the <code>:browse Modulename</code> directive. This will list all type signatures including the infered ones. |
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+ | |||

+ | |||

+ | |||

+ | [[Category:FAQ]] |
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+ | [[Category:Style]] |

## Latest revision as of 15:18, 6 February 2021

## Question

Since Haskell type checkers can automatically derive types of expressions why shall I put explicit type signatures in my programs?

## Answer

Using explicit type signatures is good style and GHC with option `-Wall`

warns about missing signatures.
Signatures are a good documentation and not all Haskell program readers have a type inference algorithm built-in.
There are also some cases where the infered signature is too general for your purposes.
E.g. the infered (most general) type for `asTypeOf`

is `a -> b -> a`

,
but the purpose of `asTypeOf`

is to unify the types of both operands.
The more special signature `a -> a -> a`

is what you want and it cannot be infered automatically.
Another example:

```
emptyString :: ShowS
emptyString = id
```

Where `ShowS`

is `String -> String`

rather than `a -> a`

.

Even more, for some type extensions the automatic inference fails,
e.g. the higher-order types used by `Control.Monad.ST.runST`

```
runST :: (forall s . ST s a) -> a
```

cannot be inferred in general, because the problem is undecidable. In GHC, they are enabled with the language pragma `RankNTypes`

.

## How to add a bunch of signatures?

Ok, this convinced me. How can I add all the signatures I did not write so far?