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shared by Javier Neira on 21 Dec 09 - Cached

First of all, it is now impossible for a function to demand a Worker having a specific type of buffer. Second, the type of foo can now be derived automatically without needing an explicit type signature. (No monomorphism restriction.) Thirdly, since code now has no idea what type the buffer function returns, you are more limited in what you can do to it.
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This illustrates creating a heterogeneous list, all of whose members implement "Show", and progressing through that list to show these items: data Obj = forall a. (Show a) => Obj a   xs :: [Obj] xs = [Obj 1, Obj "foo", Obj 'c']   doShow :: [Obj] -> String doShow [] = "" doShow ((Obj x):xs) = show x ++ doShow xs With output: doShow xs ==> "1\"foo\"'c'"
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Existential types in conjunction with type classes can be used to emulate the dynamic dispatch mechanism of object oriented programming languages. To illustrate this concept I show how a classic example from object oriented programming can be encoded in Haskell.
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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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...1 more annotation...
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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