Noob - Getting Started Infinte Loop?
Thu Aug 31 03:03:21 CEST 2006
>ke han wrote:
>>3 - Also, for efficiency sake, you should name the variable "First"
>>as "_First" to indicate you do not want the value bound.
>I was under the impression so far that all variables in Erlang are bound,
>unless 'bound' means something other than you can't change the value of a
>variable after assignment? Thanks!
There are actualy two uses of of the _ (underscore).
* case 1; as a way to tell pattern matching to match anything (and don't
bind anything to the _) e.g. in a function clause:
foo(_, V2) -> V2; % ignore the first argument, as it's unused in this
foo(...) -> ...
* case 2; as a way to tell the compiler that it is ok that the variable
name V is unused (you'll otherwise get a "variable V unused in ..."
error when using erlc) e.g.:
foo(_V) -> ok; % variable V is unused, renamed to _V to inidicate that
it's not used in this clause
foo(V) -> ....
Note that _V is still a valid variablename, so the code below is valid:
_V * 2.
a call to foo(42) would yield 84 in this case.
But you should never use _ in variable names in this way (instead remove
the _ if the V is to be used).
The _ is usually applied to variable names after the compiler complains
about unused variable names and we have ensured that the lack of usuage
isn't due to some kind of bug - e.g. we forgot to use the variable or
accidently instead used another variable in the wrong place.
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