What does `... | a b -> c` in typeclass declaration mean? [duplicate]

Question

Possible Duplicate:
What’s the “|” for in a Haskell class definition?

In the following (from Data.HList.HListPrelude, how do I interpret the | l l' -> l'' part?

class HAppend l l' l'' | l l' -> l''
 where
  hAppend :: l -> l' -> l''

And is it a part of standard Haskell (i.e. Haskell 2010) or an extension?

Answer 1

It is an extension called Functional Dependencies which you can spot in future by looking at the top of the file.

It essentially says l'' is uniquely determined from l and l'.

It is required because HAppend is a multiparameter typeclass (which is also an extension).