[erlang-questions] : abstract patterns vs macros was:( Floating guard sequences)
Mon Feb 23 01:49:56 CET 2009
On 21 Feb 2009, at 12:01 am, Zvi wrote:
> I like Abstract Patterns, I think they make patterns more like first-
> citizens and make ADTs implementation easier. There several
> comments, which
> I'll write later. For now, I just trying to write simple abstract
> for inclusive range of integers:
> #range(N,M) when is_integer(X) -> N=<X, X<=M.
> I just introduced X. Is this right way?
An abstract pattern lets you name and reuse a combination of
pattern match and type test that you could have written without it.
The form of a pattern definition is
#<head> when <guard> -> <pattern>.
In this case, N =< X, X =< M is a guard, not a pattern.
I think what you want is
#range(L, U, X)
when is_integer(X), L =< X, X =< U
Without abstract patterns, you would write
f(..., X, ...) when is_integer(X), L =< X, X =< U ... ->
The bit you would have written in the head of a function clause
goes on the right of the arrow, and the bit that would have
been in the guard of a function clause goes in the guard of the
> Do I need to specify is_integer(N), is_integer(M) for abstract pattern
> arguments too ?
Only if you want to. The one thing you cannot do is solve for
the bounds. Any variable in the head of a pattern that does not
occur in the right hand side must be bound some other way.
So you can do
f(N, #range(1,N, X)) -> ...
or f(N, #range(1,N, 42)) -> ...
g(#range(1,N, 42)) -> ... N ...
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