functional composition in haskell

Question

How come I can't do ``` fst . fst (("Bob",12),10) ``` in Haskell? ``` :t fst . fst Prelude> ((c,b),b1) -> c ``` Doesn't this make (("Bob",12),10) a good candidate for fst . fst since it's ``` (([Char],Integer),Integer) ```

Accepted Answer

The highest precedence in Haskell is function application or `f a`. So

```
fst . fst ((a, b), a)
```

is parsed as

```
fst . (fst ((a, b), a))
```

which is obviously nonsense. You can fix this with the `$` operator which is just function application with the lowest precedence, so `f $ a == f a`.

```
fst . fst $ ((a, b), a)
```

Or with some parens

```
(fst . fst) ((a, b), a)
```

functional composition in haskell

Solution

Problem