SICStus Prolog中的create_mutable / 2

Question

The SICStus Prolog manual page on mutable terms states that:

[...] the effect of unifying two mutables is undefined .

Then, why does create_mutable(data,x) fail?

Shouldn't that rather raise an uninstantiation_error ?

I cannot think of a situation when above case is not an unintentional programming error ( X vs x )... please help!

Answer 1

The short answer to "Why does create_mutable/2 not throw an exception when output unification fails?" is just: Because this was how it was done when the feature was added to SICStus Prolog, and no one has made a strong case for changing this.

One important "difference between the stream created by open/4 and the mutable term created by create_mutable/2 " is that open/4 has side-effects that are not undone if the output-unification of the call to open/4 fails.

In this sense, create_mutable/2 is somewhat more like is/2 which also just quietly fails if the output argument is some non-numeric non-variable term, eg in x is 3+4 . This seems to be the common, and traditional, way of handling output arguments in Prolog.

I agree that a non-variable as second argument is most likely a programming error. The next version of the SICStus IDE, SPIDER , will warn for this (as it already does for is/2 ).

None of this, nor the example in the question, seems directly related to the cited documentation "[...] the effect of unifying two mutables [...]".