Question

m = [[1,0,0],[2,-3,0],[4,5]]
t all@(x:xs) = let (m, n) = (length all, length x) in all (==n) (map length all)

The command:

t m

gives:

Couldn't match expected type `(Int -> Bool) -> [Int] -> t'
       against inferred type `[[a]]'
In the expression: all (== n) (map length all)
In the expression:
    let (m, n) = (length all, length x) in all (== n) (map length all)
In the definition of `t':
    t (all@(x : xs))
           = let (m, n) = ... in all (== n) (map length all)

Answer 1

The reason is, that you redefine the symbol all to the parameter of t. Thus, the local all shadows the global all and you get this error. As a solution, try to give your local all another name.