Simon Guest asked:
> Please could someone explain the meaning of | in this class declaration (from
>Andrew's example):
>
> class (Ord k) => Map m k v | m -> k v where
> lookupM :: m -> k -> Maybe v
>
> I couldn't find reference to this in any of my standard Haskell tutorials, nor the
>Haskell 98 report. Any references?
It might not have become clear from the previous answers:
this construction is not Haskell 98, but an extension.
That's why it's not in the report.
BTW: Already multi parameter type classes
(such as "class (Ord k) => Map m k v where ...")
^ ^ ^ only one type variable allowed here
aren't Haskell 98.
All the best
Christian Sievers
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