russ.abbott at gmail.com
Thu Sep 30 23:54:50 EDT 2010
In explaining fromIntegral, I want to say that it is really a collection of
Integer -> Double
Int -> Float
When GHC compiles a line of code with fromIntegral it in, it must decide at
compile time which of these overloaded functions to compile to. This is in
contrast to saying that fromIngetral is a function of the type (Integral a,
Num b) => a -> b. In reality there is no (single) function of the type
(Integral a, Num b) => a -> b because (among other things) every function
must map between actual types, but (Integral a, Num b) => a -> b does not
say which actual types are mapped between.
Is the preceding a reasonable thing to say?
If so, can I say the same sort of thing about constants like 1 and ? In
particular there is no single value . Instead  is a symbol which at
compile time must be compiled to the empty list of some particular type,
e.g., [Int]. There is no such Haskell value as  :: [a] since [a] (as
type) is not an actual type. I want to say the same thing about 1, i.e.,
that there is no such Haskell value as 1 :: (Num t) => t. When the symbol 1
appears in a program, the compiler must decide during compilation whether it
is intended to be 1::Int or 1::Integer or 1::Double, etc.
-- Russ Abbott
Professor, Computer Science
California State University, Los Angeles
Google voice: 424-242-USA0 (last character is zero)
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Beginners