I'm a bit unhappy with variables and functions.
Functions in Felix may not have side effects, but they're not
referentially transparent because they can still depend on
variables in their context.
I'd like to split them up into pure functions and the other kind.
fun ... pure
method .. may depend on variables
Neither can modify their environment.
There's a similar problem with "val" style variables (only worse);
A val takes on the value of its initialiser when control passes through it,
so it can depend on variables. The value cannot change, but it does
depend on when the val is initialised.
Worse, you can currently initialise a val twice, and, you can refer to it
before it is initialised:
var z = x; // not init yet
var y = 1;
label:>
val x = y; // multiple initialisations
++y;
goto label;
I think I would like:
const x = expr; // may not depend on variables
Reference before use and multiple initialisation can be
prevented for consts, by evaluation them all at the top
of a function in order of dependencies (topological sort).
A const cannot depend on a val parameter.
This is because vals can be lazily evaluated, and so
have a different value depending on when their initialising
expression is evaluated. Hmmm. In fact because of this
indeterminacy, you're not supposed to pass expressions
to vals which contain generators. interestingly in functions
(or methods) its safe to pass expressions depending on
variables! This is because during execution, the value
of the variable cannot change.
HOWEVER if a closure is formed, all bets are off.
We don't know when the closure will be executed so
the variable could change between the point of closure
formation and execution.
This is a know problem in Felix (its NOT a bug, but its
hard to deal with:
for var i in 0 upto 10 do
spawn_fthread { println$ i; };
done
True the fthread is a procedure, but the point remains that
this exact problem has bitten almost everyone: all the fthreads
can print 10. you cannot fix this with a closure either,
not even my passing i as an argument:
proc f (y:int) () { println$ y; }
for var i in 0 upto 10 do
spawn_fthread ( f x );
done
Doesn't help, because proc f will be inlined.
It follows const expressions can only depend on consts
and funs, not vals, vars, methods, or generators.
[marking f noinline fixes it, but that's an implementation detail!]
Unfortunately const is already used:
const x : int = "x";
so I propose to change that to
expr x : int = "x";
because actually the RHS can be any expression.
in fact this is just an argument-less version of
fun f: unit -> int = "x";
--
john skaller
skal...@users.sourceforge.net
http://felix-lang.org
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