[erlang-questions] Why can't you have a variable before a string with pattern matching?
Thu May 31 22:11:24 CEST 2007
Javier =?iso-8859-1?q?Par=EDs?= <> wrote:
> El Thursday 31 May 2007 19:37:14 Jordan Wilberding escribió:
> > Hello,
> > I was just wondering why having
> > fcn("text" ++ String) -> ..
> > is legal, but doing
> > fcn(String ++ "text")
> > is not legal. Is this for performance reasons, or something else?
> This is an speculation, but it's probably because it's easy for the compiler
> to see that the first pattern is time-bounded, while the second is not
> (because it depends on the lenght of the string).
Matching already can take that much time, e.g.
fcn(A, A) -> ...
However, the second pattern above requires allocating new structures
on the heap, while the first one shares structure with the argument. I
don't know if that's the actual rationale. Anyway, trying to make ++
fully general in patterns would open up possibilities like
fcn(A ++ "x" ++ B).
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