[Haskell-cafe] Assignment, Substitution or what?

ok ok at cs.otago.ac.nz
Tue Oct 2 22:27:42 EDT 2007

On 3 Oct 2007, at 1:42 pm, PR Stanley wrote:
> When a function is declared in C the argument variable has an  
> address somewhere in the memory:
> int f ( int x ) {
> return x * x;
> }

Wrong.  On the machines I use, x will be passed in registers and will
never ever have an address in memory.  In fact, unless I hold the Sun
C compiler back by main force, it will probably open code f() so that
there isn't any f any more, let alone any x.  Here's the actual SPARC
code generated for f:

	f:	retl
		mulx	%o0,%o0,%o0

No memory, no address.

In C, a function call is rather like entering a new block, declaring
some new variables (which might or might not ever go into memory) and
initialising them to the values of the arguments.

In Haskell, carrying out a function call is rather like creating some
new names and *binding* them to promises to evaluate the arguments
(if they are ever needed) and then returning a promise to evaluate the
body (if it is ever needed).  The key point is *binding* names to
(deferred calculations of) values, rather than *initialising*
mutable variables to (fully evaluated) values.  How this is done is
entirely up to the compiler.

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