Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!cs.utexas.edu!turpin
From: turpin@cs.utexas.edu (Russell Turpin)
Newsgroups: comp.lang.fortran
Subject: Re: Standard conformance
Summary: Now that would be a truly wonderful compiler.
Message-ID: <7002@cs.utexas.edu>
Date: 6 Oct 89 20:25:14 GMT
References: <1301@tukki.jyu.fi> <612@mbph.UUCP>
Organization: U. Texas CS Dept., Austin, Texas
Lines: 17

In article <612@mbph.UUCP>, hybl@mbph.UUCP (Albert Hybl  Dept of Biophysics  SM) writes:
> It also contains another serious error; the variable I is used
> in an expression before it is assigned a value.  Is that legal
> in X3.9-1978 fortran or is it just another allowed extension?
> Are there any implementations that detect this type of error?
> Why aren't all standard conforming compilers required to detect
> and report such errors? ...

Some compilers will tell you, in the obvious cases, that a
variable is used before it is assigned a value.  But determining
in general whether or not every (or any or no) run of a program
will use a variable before it has assigned the variable a value
is impossible.  (Exercise for reader: reduce to the halting
problem.) Like array subscript errors, this kind of problem can
only be fully detected at run-time in the cases in which it occurs.

Russell