[Haskell-cafe] arr f

Henrik Nilsson nhn at Cs.Nott.AC.UK
Fri Jan 27 15:47:53 EST 2006

Hi Sven,

 > since this is my first post,
 >    "Hello all"


 > I have a problem with a function taken from class Arrow.
 >    arr :: (b -> c) -> a b c
 > To build my own arrow, i need to distinguish between different kinds
 > of b or c. For instance, if b has the form (d,e), i want to construct
 > something different as when having form (d,e,f).

I'm not quite sure I understand what you mean.

Are you saying that you you want the input to be either a pair or a

Then one possibility would be to wrap up the input in the standard
Haskell type "Either".

For reference, "Either" is defined like this:

     data Either a b = Left a | Right b


    g :: T1 -> T2 -> T
    h :: T1 -> T2 -> T3 -> T

for some specific types T1, T2, T3 and T.

Now you can define a function "f":

    f :: Either (T1,t2) (T1,T2,T3) -> T
    f (Left  (x,y))   = g x y
    f (Right (x,y,z)) = h x y z

Thus "f" "distinguish[es] between different kinds" of input.
If it is applied to what essentially is a pair, it will compute
a result of type "T" using the function "g", whereas if it is
applied to what essentially is a triple it will compute
a result using the function "h", again of type "T".

Lifting "f" into an arrow yields:

    arr f :: Arrow a => a (Either (T1,T2) (T1,T2,T3)) T

I don't know if that was what you really ment by "construct[ing]
something different" depending on the input type?

Hope that helps,


Henrik Nilsson
School of Computer Science and Information Technology
The University of Nottingham
nhn at cs.nott.ac.uk

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