Iterator with openArray parameter make the argument evaluated many times

[slangmgh]

When using openArray type in iterator, if the argument is function call, it will be evaluated many times.

Example

proc p1(): seq[int] =
   echo "next"
   @[1,2]

iterator ip1(v: openArray[int]): auto =
   for x in v:
      yield x

for x in ip1(p1()):
   echo x

Current Output

next
next
1   
next
next
2   
next

Expected Output

next
1
2

$ nim -v
Nim Compiler Version 1.1.1 [Windows: i386]
Compiled at 2020-01-27
Copyright (c) 2006-2019 by Andreas Rumpf

active boot switches: -d:release

[metagn]

The issue #2563 covers this

[krux02]

one could argue that this is a duplicate. But personally I don't like to close this issue, because it provides a nice example and follows the issue template correctly unlike the issue mentioned above.

[krux02]

There is a workaround for this bug:

proc p1(): seq[int] =
   echo "next"
   @[1,2]

iterator ip1(v: openArray[int]): auto =
   for x in v:
      yield x

for x in ip1((discard;p1())):
   echo x

Yea, don't ask me why this works.

[timotheecour]

proper fix is making openArray first class, see nim-lang/RFCs#88