# Type inference

**Type inference** is a feature of the type system which means that concrete types are deduced by the type system whereever it is obvious.
If you add an integer variable `x`

to a numeric literal `2`

,
then the type system concludes that `2`

, which in principle can represent 2 for every number type, must also be an integer,
since `+`

supports only addition of numbers of the same type.
(This restriction is a good thing, as we explain for the idea of a Generic number type.)

Another example: There are the following standard functions:

```
map :: (a -> b) -> [a] -> [b]
Char.ord :: (Char -> Int)
```

For the expression `map ord`

the type checker finds out that the type variable `a`

must be bound to the type `Char`

and `b`

must be bound to `Int`

and thus it concludes

```
map ord :: [Char] -> [Int]
```

You can play with the type inference mechanism in Hugs and GHCi. See Determining the type of an expression.

## See also

The type inference mechanism is very similar to unification in PROLOG.

- Haskell-Cafe: Learn Prolog ...