http://www.muitovar.com/monad/moncow.xhtml#list
contains a cross function which calculates the cross product
of two lists. That attached does the same but then
used cross on 3 lists. Naturally, I thought use of
fold could generalize that to n lists; however,
I'm getting error:
{-- cut here
Compilation started at Sun Nov 23 13:15:24
make
runghc -XMultiParamTypeClasses -XFunctionalDependencies
-XFlexibleInstances cross.hs
cross.hs:23:30:
Occurs check: cannot construct the infinite type: a = (a, a)
Expected type: [a] -> [a] -> [a]
Inferred type: [a] -> [a] -> [(a, a)]
In the first argument of `Data.Foldable.foldl1', namely `cross'
In the first argument of `print', namely
`(Data.Foldable.foldl1 cross (list2, list3, list1))'
make: *** [run] Error 1
}-- cut here
How do I apply fold to cross and n lists, for n>=0?
TIA.
-regards,
Larry
{-
Purpose:
Demonstrate http://www.muitovar.com/monad/moncow.xhtml#list
-}
import Data.Foldable
cross::[a]->[b]->[(a,b)]
cross ls1 ls2 = do
{ x<- ls1
; y<- ls2
; return (x,y)
}
list1=[1,2]
list2=[10,20]
list3=[100,200]
main = do
putStrLn "cross list2 list3="
print (cross list2 list3)
putStrLn "cross list1 (cross list2 list3)="
print (cross list1 (cross list2 list3))
print (list2,list3,list1)
print (Data.Foldable.foldl1 cross (list2,list3,list1))
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