Fail in easy list manipulation

Question

Using GHCi I do the following : ``` prelude> let lol [] = [] prelude> let lol (x:xs) = (lol xs) ++ [x] ``` When I try to evaluate ``` prelude> lol [1, 2, 3] ``` I get ``` Exception: <interactive>:3:5-32: Non-exhaustive patterns in function lol ``` I think I understand the problem (list with 1 element not matched ?) but can't see why he can't match x:xs as x:[]

Accepted Answer

```
prelude> let lol [] = []
```

defines a function of type `[a] -> [b]` that will produce a pattern-match failure when passed a non-empty list. That definition is then shadowed by

```
prelude> let lol (x:xs) = (lol xs) ++ [x]
```

of type `[a] -> [a]`, which will cause a pattern-match failure when its argument is an empty list.

`let` bindings are not incremental, a new binding for a name shadows the old binding.

You can define a function with several clauses by separating the clauses with a semicolon,

```
let lol [] = []; lol (x:xs) = lol xs ++ [x]
```

Fail in easy list manipulation

Solution

Problem