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Re: Instruction return
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Date: 1997-06-02 (21:49)
From: Pierre Weis <Pierre.Weis@i...>
Subject: Re: Instruction return
> This is one of the two main control problems with functional programming,
> The solution I use is:
Dwight's solution uses the a user defined exception Found_it instead of the
predefined exception Exit, with a loop or an iteration functional.

> The other control problem is when you want the equivalent of:
>     x = foo(a);
>     bar();
>     return x;
> If you try 
>     let x = foo(a) in
>     bar();
>     x
> then bar gets done first, instead of second.

That's not true: the language semantics ensures that foo (a) is
evaluated before the sequence bar (); x. (This is intuitively evident
since the value of x may be required to evaluate the "in" part.) 

(Try for instance:
 let foo x = print_string "Hello "; print_string x; x;;
 let bar () = print_string " world!";;
And then
# let x = foo ("you") in bar(); x;;
Hello you world!- : string = "you")

>  i use this instead:
>     let f x = bar(); x in	
>     in f (foo a)

This is provably equivalent, although slightly offuscating.

(Theory meets practice in this case, since we get:
# let f x = bar(); x in f (foo "you");;
Hello you world!- : string = "you")

> I haven't found any other serious problems with Caml's logic flow.  These
> two just take some getting used to.

Good. Especially if we discarde the second problem :)

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/