Re: [Caml-list] Strange behavior of mutualy recursive definitions

[Guillaume Yziquel <guillaume.yziquel@citycable.ch>]

Le Wednesday 27 Apr 2011 à 22:46:29 (+0200), rixed@happyleptic.org a écrit :
> I met this strangeness today and would like to know is it a FAQ? Is it
> intentional? Is it fixable?
> 
> # let inc f x = (f x)+1 and dec f x = (f x)-1;;
> val inc : ('a -> int) -> 'a -> int = <fun>
> val dec : ('a -> int) -> 'a -> int = <fun>
> # let rec toto = inc titi and titi = dec toto;;
> Error: This kind of expression is not allowed as right-hand side of `let rec'
> 
> Now after reading http://caml.inria.fr/pub/docs/manual-ocaml/manual021.html#s:letrecvalues ,
> this is unclear why toto and titi definitions are not 'statistically constructive':
> It seams that one could trivially add a "fun f -> ... f" around the
> function bodies in order to comply with the rules. And actually, this
> even simpler (but to my knowledge equivalent) definition works :
> 
> # let rec toto x = inc titi x and titi x = dec toto x;;
> val toto : 'a -> int = <fun>
> val titi : 'a -> int = <fun>
> 
> So why was the first version rejected?

Because to evaluate toto, you need to have titi evaluated beforehand
(wouldn't be the case if you had 'let rec toto = titi inc' instead of
'inc titi', typically). And to evaluate titi, you need to have evaluated
beforehand the value toto.

The second version only defines toto as a function that calls titi. No
need to have titi evaluated to be able to evalue toto. You only need to
know where it is declared. That's the main difference.

-- 
     Guillaume Yziquel