Am Freitag, 7. Oktober 2005 06:32 schrieb Cale Gibbard:
That | m -> s reads "where m determines s", and means that there can be at most one s (here, the state type) for a given m (the monad type). This is used in type inference and checking to ensure that the type of state being carried around by a monad in MonadState is well defined, and to prevent you from having to annotate the operations in MonadState with type signatures. (To tell which type s to use.)
What you can express via this syntax is called functional dependencies (in analogy to functional dependencies in database theory). One should note that functional dependencies and even multi-parameter type classes are an extension to Haskell 98 and not part of Haskell 98 which is why you haven't read about them in the tutorial.
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Best wishes, Wolfgang