[erlang-questions] Reassigning variables

[Richard O'Keefe]

On 18 Mar 2009, at 7:07 pm, Matthew Dempsky wrote:
> Also, here's a list of a few other functions in OTP that use numbered
> variables:
>
>  - zip:put_z_files/6
>  - epp:predef_macros/1

predef_macros(File) ->
     Ms0 = dict:new(),
     Ms1 = dict:store({atom,'FILE'}, {none,[{string,1,File}]}, Ms0),
     Ms2 = dict:store({atom,'LINE'}, {none,[{integer,1,1}]}, Ms1),
     Ms3 = dict:store({atom,'MODULE'}, undefined, Ms2),
     Ms4 = dict:store({atom,'MODULE_STRING'}, undefined, Ms3),
     Ms5 = dict:store({atom,'BASE_MODULE'}, undefined, Ms4),
     Ms6 = dict:store({atom,'BASE_MODULE_STRING'}, undefined, Ms5),
     Machine = list_to_atom(erlang:system_info(machine)),
     Ms7 = dict:store({atom,'MACHINE'}, {none,[{atom,1,Machine}]}, Ms6),
     dict:store({atom,Machine}, {none,[{atom,1,true}]}, Ms7).

This really doesn't need them.  It would be better as

predef_macros(File) ->
     Machine = list_to_atom(erlang:system_info(machine)),
     dict:from_list([
	{{atom,'FILE'}, 	      {none,[{string,1,File}]}},
	{{atom,'LINE'},		      {none,[{integer,1,1}]}},
	{{atom,'MODULE'},	      undefined},
	{{atom,'MODULE_STRING'},      undefined},
	{{atom,'BASE_MODULE'},	      undefined},
	{{atom,'BASE_MODULE_STRING'}, undefined},
	{{atom,'MACHINE'},	      {none,[{atom,1,Machine}]}}.
	{{atom,Machine},	      {none,[{atom,1,true}]}
     ]).

There's a tiny performance loss, but since it's done once per file,
it's not worth worrying about.