[erlang-questions] illegal guard expression: filelib:is_dir(H)
Sun Feb 14 12:11:43 CET 2010
Oops, put that first example after the second (not the third)
paragraph. That's what I get for mailing from my phone. :(
Sent from my iPhone
On Feb 14, 2010, at 3:09 AM, Jayson Vantuyl <kagato@REDACTED> wrote:
> Oddly, in Erlang, "if" only accepts expressions that are valid in
> guards. This usually means special builtin functions that are
> normally used to make pattern matching more precise. Because pattern
> matching must be fast, most function calls are forbidden.
> For this reason, you'll usually see a case (or a try) statement
> where you'd expect an "if".
> To make thing even more confusing, "if" is most useful when you have
> a number of unrelated comparisons (limited to guards) that should be
> checked until one is true. Most other languages would use a chain of
> if-elsif-else statements or even a case statement. E.g. "case
> filelib:is_dir(H) of true -> ...; false -> ... end"
> To top it off, you could save that expression in a variable and that
> could work, but would still be confusing. E.g. "F = filelib:is_dir
> (H), if F == true -> ...; true -> ... end"
> This is all a but backwards and probably more confusing than it has
> a right to be. That said, you should probably read the section of
> the Erlang manual about guards. Once you grok them, they're
> powerfully useful. Until then, just remember that "if" is for guards
> only, and you probably want a case (or try) statement.
> Good luck!
> Sent from my iPhone
> On Feb 14, 2010, at 2:51 AM, Kay Kay <kaykay.unique@REDACTED> wrote:
>> I have a function along the following lines
>> do_process_elements(, _) -> ;
>> do_process_elements([H | _], Fn) ->
>> filelib:is_dir(H) ->
>> process_directory(H, Fn);
>> true ->
>> process_file(H, Fn)
>> When I compile - I get the following error, where the line number
>> refers to - filelib:is_dir(H) function.
>> ./test.erl:23: illegal guard expression
>> Given that filelib:is_dir(H) returns true / false - I thought this
>> would fit in here as a predicate. What could be going wrong ?
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