On Tue, Sep 13, 2011 at 00:08, Vasili I. Galchin
I am trying to model multigraphs ....but getting errors with ghci and can't figure out why.... I have a serious blind spot ....
Why do you need to use classes for this? (Note: forget everything you know about classes from OOP. Haskell typeclasses have approximately nothing to do with OOP.) junk1.hs:19:12:
Constructor `Arrow' should have 1 argument, but has been given 0 In the pattern: Arrow In the definition of `source': source Arrow = fst Arrow In the instance declaration for `Graph Arrow Int'
It's asking "Arrow *what*?" You specified Arrow as taking a tuple argument; if you want to use it here, you need to provide that argument (or a placeholder, but in this case you clearly want the tuple).
source (Arrow p) = fst p target (Arrow p) = snd p
Or you can use pattern matching to deconstruct the tuple as well:
source (Arrow (f,_)) = f target (Arrow (_,t)) = t
-- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms