However, I didn't understand the syntax of the class declaration.
Lemmih wrote:
> import Complex
>
> class ConvertibleToComplex a b | a -> b where
> toComplex :: RealFloat b => a -> Complex b
I've looked at several sources to try to understand this declaration. I
can't find any examples where a class declaration takes two type
variables or uses the pipe symbol. One of the sources I used was
_Haskell 98 Language and Libraries The Revised Report_. I'm not an
expert at BNF notation, but the definitions for topdecl, tycls, and
tyvar in the grammar seems to exclude Lemmih's declaration.
If someone could point me to some sources that explain this notation,
I'd be very grateful.
This is a multi-parameter typeclass (with functional dependencies),
which is not a part of Haskell 98, so its no surprise that you didn't
find it in the Report. The Muli-parameter part is pretty easy to
understand: the type class simply introduces more than one type
variable. Functional dependencies let you say which types should
determine which other types. Here are some pages on these ideas:
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#multi-param-type-classes
http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#functional-dependencies
http://haskell.org/hawiki/FunDeps
In this particular instance, what the class definition means is that
'ConvertaibleToComplex' is a class which relates two types 'a' and 'b'
AND the type 'a' uniquely determines the type 'b'.
Hope that helps,
Robert Dockins
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