Haskell function composition question

Question

If this works: ``` Prelude Data.Char> map toUpper ("sdfsd" ++ "dfgfdg") "SDFSDDFGFDG" ``` Then why this doesn't? ``` Prelude Data.Char> map toUpper . (++) "sdfsd" "dfgfdg" <interactive>:1:14: Couldn't match expected type `a -> [Char]' against inferred type `[Char]' In the second argument of `(.)', namely `(++) "sdfsd" "dfgfdg"' In the expression: map toUpper . (++) "sdfsd" "dfgfdg" In the definition of `it': it = map toUpper . (++) "sdfsd" "dfgfdg" ```

Accepted Answer

```
map toUpper . (++) "sdfsd" "dfgfdg"
```

is parsed as:

```
(map toUpper) . ((++) "sdfsd" "dfgfdg")
```

So basically you're doing

```
(map toUpper) . "sdfsddfgfdg"
```

This does not work because the second argument to `.` needs to be a function, not a string.

I assume you were trying to do something more like `(map toUpper . (++)) "sdfsd" "dfgfdg"`. This also does not work because the return type of `++` is `[a] -> [a]` while the argument type of `map toUpper` is `[a]`.

The thing here is that while one might think of `++` as a function that takes two lists and returns a list, it really is a function that takes one list and then returns a function which takes another list and returns a list. To get what you want, you'd need to `++` into a function that takes a tuple of two lists and returns a list. That's called uncurrying. The following works:

```
map toUpper . (uncurry (++)) $ ("sdfsd", "dfgfdg")
```

Haskell function composition question

Solution

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