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shared by Javier Neira on 24 Jan 10 - Cached

The most interesting are probably the static checks that can be done with Type arithmetic, that enforce the number of bands at compile time, rather than runtime, by lifting the band count into the type level.
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typecheck perform the check statically, using phantom types and Peano numbers.
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So encode a type-level version of the bounds check. Only resistors with bands >= 4 and <= 8 are valid:
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Further checks can be obtained by separating the metal and ceramic values on the type level, so no function that takes a metal resistor can be accidentally passed a ceramic one. A newtype is useful for this:
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shared by mountain on 24 Jun 07 - Cached

shared by Javier Neira on 09 Dec 09 - Cached

8. Functors
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A "functor" is a structured collection (or container) type with a method (fmap) that accepts a method and applies that method to the members of the collection yielding an isomorphic collection of values of a (possibly) new type. Is this right?
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Every monad is a functor, but not the other way around; a monad is a functor PLUS functions >>= and return satisfying some laws
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a functor is a type constructor PLUS a function fmap satisfying some laws.
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I think it's better to use existentials, as they let you define multiple instances for the same type.
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People tend to forget that the major difference between ADT's and OO-style classes is really only that with a class you can have many instances in the same program simultaneously, whereas with an ADT you can have only one; but the ADT implementation is still interchangeable.
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sequence :: Monad m => [m a] -> m [a]
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