[erlang-questions] Ideas for a new Erlang
Sat Jun 28 11:17:36 CEST 2008
Robert Virding writes:
> On 26/06/2008, Richard A. O'Keefe <> wrote:
> > p6. "The complex selective receive primitive of old Erlang
> > has forced the Erlang designers to introduce a more
> > restricted type of expressions called _guards_."
> > This is fiction, and it's fiction with a major error in it.
> > Guards are *NOT* a restricted type of expression.
> Richard is absolutely right here. Guards were always seen as part of
> pattern matching, the bits which you could not easily write in the
> patterns. They have always been a sequence of *tests* which either
> succeed of fail, there has never been a concept of error in guards. In
> this sense they have never been proper expressions, even restricted
> ones. Boolean operations were added to expand guards and tests the
> changed to more resemble expressions, for example tuple(T) =>
> is_tuple(T), but guard tests are not expressions, they are *tests*.
As I already explained in my reply to Richard, I meant expressions in
a general sense.
Expressions in C or C++ are expressions. So are expressions in Java,
Common Lisp and most other programming languages. Even regular
expressions are expressions. Can you give a definition of the concept
"expression" that excludes guard tests of Erlang and includes all
other types of expressions?
> It might have been the wrong to expand the tests with boolean
> operations as it means that it is more difficult to see the
> difference, and hence to understand it.
> But they are not expressions, they never have been, they are tests!
What I really wanted to say was that guard tests of Erlang are strange
and unusual. I see nothing in your posting that contradicts that.
> P.S. I have seen attempts, not in Erlang, to include the "guard" bit
> in the patterns and they were *very* hard to read.
Not sure what that means.
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