Erlang Macro Oddness
I found a little oddity with Erlang macros while I was writing version 2 of the Weighted Slope One algorithm. It seems that passing a multi-statement anonymous function as a parameter into a macro confuses the compiler.
For example, this code works:
-module(macro_oddness).
-export([start/0]).
-define(A_MACRO(FunAnon), apply(FunAnon, [])).
start() ->
?A_MACRO(
fun() ->
io:format("Single-element function works fine.~n")
end).But this code produces a compile-time error:
-module(macro_oddness).
-export([start/0]).
-define(A_MACRO(FunAnon), apply(FunAnon, [])).
start() -> ?A_MACRO(
fun() ->
io:format("Multiple-element function "),
io:format("does not compile.~n")
end).An "argument mismatch for macro ''A_MACRO''" error, to be precise.
Interestingly, multiple statements in a begin..end block seem to be okay:
-module(macro_oddness).
-export([start/0]).
-define(A_MACRO(FunAnon), apply(FunAnon, [])).
start() -> ?A_MACRO(
fun() ->
begin
io:format("Multiple-element function "),
io:format("with a begin..end block is okay.~n")
end
end).Something to keep an eye out for.
And do you know the reason of such behaviour?
ReplyDeleteQrilka: I am guessing that the Erlang preprocessor is not flagging the 'fun' keyword as the beginning of a block of code that ends with the 'end' keyword, and instead ends the first statement at the comma (the usual statement terminator).
ReplyDeleteThe first argument would then be taken as 'fun() -> io:format(...)' and the second as 'io:format(...) end', which would cause the single-argument A_MACRO macro to choke during expansion.