Is it possible to uses guards within guards?

Question

I wonder if it is possible to use guards inside guards in haskell. like this :

analyser modele instr
  |  instr == NoCom  = (modele, "No command" ,[])
  |  instr == ComK   | (read comargstr1) == 0 = function1 modele Nothing
                     | (read comargstr1) == 1 = function1 modele (Just (read comargstr1))
                     | (read comargstr1) <  0 = function1 modele (Just (read comargstr2))
                     | otherwise              = function2 modele
  | othercases...
  | othercases...

In my example, i can't evaluate (read comargstr1) in the first column of guards at all because comargstr1 doesn't always return a compatible string readable with read (fatal error)

I didn't managed to use guards within guards!

Is it possible to do it (Is there a trick, an option, something special, ...) or is it purely impossible?

Thank you in advance for your Help!

Answer 1

Layout doesn't apply to guards, so it doesn't matter how you align them.

The closest you can do is to use MultiWayIf for the second-level guards.

Answer 2

You can achieve something vaguely similar as follows:

analyser modele instr
  |  instr == NoCom = (modele, "No command" ,[])
  |  instr == ComK  = foo
  | othercases...
  | othercases...
  where foo | (read comargstr1) == 0 = function1 modele Nothing
            | (read comargstr1) == 1 = function1 modele (Just (read comargstr1))
            | (read comargstr1) <  0 = function1 modele (Just (read comargstr2))
            | otherwise              = function2 modele

Note that you will need a distinct name foo for every branch. Also, if a nested guard list does not end with otherwise (or equivalent) then control will not be transfered to the next guard in the topmost level.

Answer 3

Just either repeat both conditions, or use an if after a single guard. Two/nested guards aren't supported as far as I'm aware.