writing a function that accepts multiple input and has multiple outputs in haskell

Question

whats the correct way to write a function that can accept different input's and have different outputs

for example i'm using hmatrix and lets say i want to accept a Matrix or a Vector in my function, and the output can be a Matrix or a Vector depending on hte formula where T in the below example can be a matrix or a vector , is maybe the right tool for this?

Myfunc ::(Matrix A, Matrix/Vector T) -> Maybe(Matrix/Vector T)

Update using either mentioned below here is one possible solution

Myfunc :: Maybe Matrix Double t -> (Either Vector Double a,Matrix Double a) -> Either (Matrix Double T,Vector Double T)

Chris Taylor · Accepted Answer · 2012-07-27 13:48:57Z

Take a look at how matrix multiplication and left-divide are implemented in the source code for HMatrix.

Essentially, they define a multi-parameter type class which tells the function how to behave for different inputs, and it has a functional dependency which tells it what output is appropriate. For example, for multiplication:

{-# LANGUAGE MultiParamTypeClasses  #-}
{-# LANGUAGE FunctionalDependencies #-}

-- |The class declaration 'Mul a b c' means "an 'a' multiplied by a 'b' returns
--  a 'c'". The functional dependency 'a b -> c' means that once 'a' and 'b' are
--  specified, 'c' is determined.
class Mul a b c | a b -> c where
    -- | Matrix-matrix, matrix-vector, and vector-matrix products.
    (<>)  :: Product t => a t -> b t -> c t

-- |Matrix times matrix is another matrix, implemented using the matrix
--  multiplication function mXm
instance Mul Matrix Matrix Matrix where
    (<>) = mXm

-- |Matrix times vector is a vector. The implementation converts the vector
--  to a matrix and uses the <> instance for matrix/matrix multiplication/
instance Mul Matrix Vector Vector where
    (<>) m v = flatten $ m <> asColumn v

-- |Vector times matrix is a (row) vector.
instance Mul Vector Matrix Vector where
    (<>) v m = flatten $ asRow v <> m

Riccardo T. · Accepted Answer · 2012-08-01 22:51:53Z

2

You could either have a look at Either (I know, it's a bad joke), or, if your function has a general meaning but different implementations on different data types, you could define a typeclass.

edit: I didn't add any further details because your question isn't completely clear to me

edited Aug 1, 2012 at 22:51

answered Jul 27, 2012 at 13:43

Riccardo T.

8,9275 gold badges40 silver badges81 bronze badges

1 Comment

pyCthon Over a year ago

thanks this worked fine for me as well as the solution below!

Martinsos · Accepted Answer · 2012-07-27 13:49:47Z

1

The question is what do you want to do with your input? For example if you want to do comparison then you can say input has to be of class Ord like this:

myFunc :: (Ord a) => a -> b

Another way would be to use Either, but in that case you can have only two different data types. For example

myFunc :: Either a b -> Either c d

can accept and return different types.

answered Jul 27, 2012 at 13:49

Martinsos

1,69315 silver badges32 bronze badges

Comments

brano · Accepted Answer · 2012-07-27 13:46:03Z

-2

Another solution would be to use a list of lists [[a]]. Essentially a vector is a matrix with a single row.

answered Jul 27, 2012 at 13:46

brano

2,89421 silver badges15 bronze badges

Collectives™ on Stack Overflow

writing a function that accepts multiple input and has multiple outputs in haskell

4 Answers 4

Comments

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related