Mmmm...now I had to go look it up. David F. Place wrote:
Hi Chris,
Yes, this is just what I need to understand. Could you point me to a description of this? I couldn't find any discussion of it in the reference document. Thanks.
Cheers, David
On Jan 7, 2006, at 12:25 PM, Chris Kuklewicz wrote:
the mind-bending-thing I had to learn with Haskell is that the "let p =" creates source code *shorthand*.
This is the first time I have tried to explain this. So it will not help much... The mantra is : Bindings are not variables The best, official thing to read is section 3.12 of the Haskell98 Report: http://www.haskell.org/onlinereport/exps.html#sect3.12 It shows that "let p = foo q = bar in ..." is translated to "let (~p,~q) = (foo,bar) in ..." which is translated to "case (foo,bar) of ~(~p, ~q) -> ..." Assuming it does not refer to iteself, otherwise it becomes "let (~p,~q) = fix (\ ~(~p,~q) -> (foo,bar) ) in ... " which is translated to "case fix (\ ~(~p,~q) -> (foo,bar) ) of ~(~p, ~q) -> ..." At which point you are in section "3.13 Case Expressions" So "let p = permutation [1..n]" merely pattern matches the bound name "p" to the value that "permutations [1..n]" lazily evaluates to. In Scheme/Lisp the let would allocate a specific cell "p" that would point to the strictly evaluated result of "permutations [1..n]". This Haskell pattern binding is the key to being referentially transparent. It means there is no difference between "p" and "permutations [1..n]". Allocating a memory cell for "p" would be an observable difference, thus Haskell does no allocation. This also means you can do this: let a = 1 b = 2 c = 3 d = a+b+c in a*b*c*d without it having to allocate all those variables, because they are shorthand, NOT VARIABLES. All bindings are constant, so the compiler can substitute / inline them and never specifically allocate intermediate stoage, thus "a,b,c,d" are not allocated. This lets you break up a big function into many small expressions with sensible names, and perhaps reuse the pieces. Thus "map/foldl/filter/..." -- Chris