[m-dev.] use of require/1 and mode error?

[Fergus Henderson]

Andrew Bromage, you wrote:
> G'day.
> 
> Fergus Henderson wrote:
> 
> > > > -	% XXX use of `require' is a bug (should be a mode error)
> > > > -	{ require(laplace(X), "laplace failed\n") },
> 
> > A higher-order term (such as the first argument to require/1)
> > is not allowed to have any partially applied output arguments,
> > because (a) it might be called any number of times, and (b)
> > it might be called inside negations.  But laplace(X) does bind `X'.
> > (a) is not a problem for `any' insts, but (b) is, so it
> > should be an error.
> 
> I realise this is early Monday and I'm not thinking straight, but
> why wouldn't (some [X] laplace(X)) work?

Only because the line following the call to require/1 is

	printmat(X)

i.e. the variable is really not local.  If you locally quantify it,
then X in the call to printmat(X) will become a singleton variable,
and that will result in a mode error.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>   |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>   |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3         |     -- the last words of T. S. Garp.