haskell: type mismatch error in recursion

Question

I have such a recursive function;

elim_all :: Idx -> Idx -> Idx -> Idx -> Mat El -> Mat El
elim_all c r1b r1e r2 m
     | r1b == r1e = elim_one c r1b r2 m
     | otherwise = elim_one c r1b r2 m : elim_all c (r1b+1) r1e r2 m

elim_one function is;

elim_one :: Idx -> Idx -> Idx ->  Mat El -> Mat El
elim_one c r1 r2 m = let val1 = ((m!!r1)!!c)
                         val2 = ((m!!r2)!!c)
                         row1 = (mulr r1 val2 m)!!r1
                         row2 = (mulr r2 val1 m)!!r2
                         nrow = zipWith (-) row1 row2
                         matr = if r1 == r2
                                     then m
                                     else replacer r1 nrow m
                      in matr

When I run it, I get the following error:

    Couldn't match type ‘[El]’ with ‘Int’
    Expected type: [El]
      Actual type: Mat El
    In the first argument of ‘(:)’, namely ‘elim_one c r1b r2 m’
    In the expression:
      elim_one c r1b r2 m : elim_all c (r1b + 1) r1e r2 m

error still doesn't make sense to me. How can I fix the problem?

type of elim_one is elim_one :: Idx -> Idx -> Idx -> Mat El -> Mat El. Type of Mat El is [[]]. — yusuf
– yusuf, Commented Feb 28, 2017 at 15:22
Then shouldn't the type of elim_all be elem_all :: Idx -> Idx -> Idx -> Idx -> Mat El -> [Mat El]? — bheklilr
– bheklilr, Commented Feb 28, 2017 at 15:26
Well, then the cons operator expects one element (a list), buy you supply a list of lists? — Viktor Mellgren
– Viktor Mellgren, Commented Feb 28, 2017 at 15:26

Ingo · Accepted Answer · 2017-02-28 17:31:31Z

2

Both elim_one and elim_all compute something of type Mat E1. But whatever this might be, since

(:) :: a -> [a] -> [a]

and for all types x, it holds that x is not the same as [x] you can never relate the results of evaluation of elim_one and elim_all with the (:) operator.

answered Feb 28, 2017 at 17:31

Ingo

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John F. Miller · Accepted Answer · 2017-02-28 17:31:18Z

2

So here is the line in question:

| otherwise = elim_one c r1b r2 m : elim_all c (r1b+1) r1e r2 m

Now you have said in your type signature that the result* of elim_all will be a Mat El, but in this line the result is a list (that is what the (:) operator forms).

Without knowing more about what the Mat type does, my best guess is that you need to wrap the output of this case in a Type Constructor of Mat.

* When the function is fully applied.

answered Feb 28, 2017 at 17:31

John F. Miller

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