Re: [Haskell-cafe] Haskell 101 on classes .... duh ..... :^)

Hi Brandon, Here is Ivan's web site. http://ivanmiljenovic.wordpress.com/ ...... I have written in Haskell before ... a POSIX Realtime hackage package. It is true I am little rusty now .... because industry is too stubborn except in places like Silicon Valley, Wall Street, Virginia (defence industry) to use FPLs ... have to pay the bills sadly... Vasili On Tue, Sep 13, 2011 at 9:57 AM, Brandon Allbery wrote:

On Tue, Sep 13, 2011 at 00:08, Vasili I. Galchin wrote:

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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:

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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).

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source (Arrow p) = fst p target (Arrow p) = snd p

Or you can use pattern matching to deconstruct the tuple as well:

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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