From 205405f0bf1d2fa37d4c8170c11689a2937f5d9c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 1 Mar 2016 06:23:36 +0100
Subject: Generalize bit string comprehensions

The expression in a bit string comprehension is limited to a
literal bit string expression. That is, the following code
is legal:

  << <<X>> || X <- List >>

but not this code:

  << foo(X) || X <- List >>

The limitation is annoying. For one thing, tools that transform
the abstract format must be careful not to produce code such as:

  << begin
      %% Some instrumentation code.
      <<X>>
     end || X <- List >>

One reason for the limitation could be that we'll get
reduce/reduce conflicts if we try to allow an arbitrary
expression in a bit string comprehension:

  binary_comprehension -> '<<' expr '||' lc_exprs '>>' :
	{bc,?anno('$1'),'$2','$4'}.

Unfortunately, there does not seem to be an easy way to work
around that problem. The best we can do is to allow 'expr_max'
expressions (as in the binary syntax):

  binary_comprehension -> '<<' expr_max '||' lc_exprs '>>' :
	{bc,?anno('$1'),'$2','$4'}.

That will work, but functions calls must be enclosed in
parentheses:

  << (foo(X)) || X <- List >>
---
 lib/stdlib/src/erl_parse.yrl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'lib/stdlib')

diff --git a/lib/stdlib/src/erl_parse.yrl b/lib/stdlib/src/erl_parse.yrl
index b1c574ea60..6f8e5e8449 100644
--- a/lib/stdlib/src/erl_parse.yrl
+++ b/lib/stdlib/src/erl_parse.yrl
@@ -311,7 +311,7 @@ bit_size_expr -> expr_max : '$1'.
 
 list_comprehension -> '[' expr '||' lc_exprs ']' :
 	{lc,?anno('$1'),'$2','$4'}.
-binary_comprehension -> '<<' binary '||' lc_exprs '>>' :
+binary_comprehension -> '<<' expr_max '||' lc_exprs '>>' :
 	{bc,?anno('$1'),'$2','$4'}.
 lc_exprs -> lc_expr : ['$1'].
 lc_exprs -> lc_expr ',' lc_exprs : ['$1'|'$3'].
-- 
cgit v1.2.3