Hi Andrew, Andrew Coppin wrote:
Now, what about type variables? What do they do? Well now, that seems to be slightly interesting, since a type variable holds an entire type (whereas normal program variables just hold a single value), and each occurrance of the same variable is statically guaranteed to hold the same thing at all times. It's sort of like how every instance of a normal program variable holds the same value, except that you don't explicitly say what that value is; the compiler infers it.
What do you mean by "hold the same thing at all times"? Consider the following program: id :: forall a . a -> a id x = x call1 :: Bool call1 = id True call2 :: Int call2 = id 42 This program contains a type variable a, and a value variable x. Now, these variables do *not* mean the same thing at all times. In the first call of id, a is Bool and x is True; but in the second call of id, a is Int and x is 42. If these variables would mean the same thing at all times, I would expect them to be called constants, wouldn't you? Tillmann