I'm going through the 99 Haskell problems to build my proficiency with the language. On problem 7 ("Flatten a nested list structure"), I found myself wanting to define a conditional behavior based on the type of argument passed to a function. That is, since

*Main> :t 1
1 :: (Num t) => t
*Main> :t [1,2]
[1,2] :: (Num t) => [t]
*Main> :t [[1],[2]]
[[1],[2]] :: (Num t) => [[t]]

(i.e. lists nested at different levels have different data types) it seems like I should be able to write a function that can read the type of the argument, and then behave accordingly. My first attempt was along these lines:

listflatten l = do
    if (:t l) /= ((Num t) => [t]) then
        listflatten (foldl (++) [] l)
        else id l

But when I try to do that, Haskell returns a parse error. Is Haskell flexible enough to allow this sort of type manipulation, do I need to find another way?