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

(mention rank n types) |
(type inference not always decidable) |
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Where <hask>ShowS</hask> is <hask>String -> String</hask> rather than <hask>a -> a</hask>. | Where <hask>ShowS</hask> is <hask>String -> String</hask> rather than <hask>a -> a</hask>. | ||

− | + | Even more, for some type extensions the automatic inference fails, | |

− | + | e.g. the higher-order types used by <hask>Control.Monad.ST.runST</hask> | |

− | |||

<haskell> | <haskell> | ||

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

## Revision as of 10:40, 25 December 2008

## 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?