The spirit of declarative programming lies in the fact that you try to just tell the compiler what something means. Instead of thinking about keeping track of the numbers of parentheses, and then applying them afterwards, I would suggest you create a function to do the same job:

    bracketed :: String -> String

    bracketed s = "(" ++ s ++ ")"

This would allow you to remove the numeric argument, and also write cleaner code.

The second thing that you can improve is the pattern matching. You can use isPrefixOf to check functions, or use a take inside a case statement to get cleaner handling of different cases.

    case take 3 str of

      "sin" -> ...

      "cos" -> ...

       _ -> ...

This would also let you debug your code more easily, just as you require.

On 4 November 2015 at 14:51, Imants Cekusins <imantc@gmail.com> wrote:

> a function that is supposed to add parenthesis to sin/cos expressions.
a) Sin 5 + 3        ->    Sin(5) + 3
b) Sin Cos (5+3)   ->    Sin (Cos(5+3))
c) Sin Cos 5 * 3   ->    Sin(Cos(5)) * 3

are you looking for textual representation or actual Haskell code which runs?
if actual code, these examples may be re-written like this:
a) sin 5 + 3
b) sin $ cos $ 5 + 3
c) sin (cos 5) * 3
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--

Regards

Sumit Sahrawat

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