top level defer is executed when it encounters import

[jiro4989]

NG Example

import streams

var s = "test.dat".newFileStream fmWrite
defer: s.close
s.write '\x61'

# i think that here 'import' is problem.
import sequtils

for d in [1, 2, 3].mapIt($it):
  s.write d

Current Output

nim c -r defer.nim   

Traceback (most recent call last)
defer.nim(10)            defer
streams.nim(136)         write
streams.nim(112)         writeData
streams.nim(421)         fsWriteData
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
Error: execution of an external program failed: '/tmp/defer '

OK Example

import streams
import sequtils

var s = "test.dat".newFileStream fmWrite
defer: s.close
s.write '\x61'

for d in [1, 2, 3].mapIt($it):
  s.write d

Output

Success executing.

Other close Example

var f = "test.txt".open fmReadWrite
defer: f.close

import sequtils

# Error is not occured.
# and "1" is not written.
f.write "1"
echo "end"

Output

$ nim c -r open.nim
end
$ cat test.txt | wc  
      0       0       0

Possible Solution

I think that executing 'import' then defer s.close is executed too.
This problem can be avoided, write imports at the beginning of the file.

Is it bad for compiler that writing the 'import' in the middle of file?

[krux02]

I think the top level defer is causing the problem. For this use case you don't technically need the defer. When the process terminates the operating system will reliably close all remaining open file handles for you.

[nc-x]

import streams

var s = "test.dat".newFileStream fmWrite
defer: s.close
s.write '\x61'

echo s.f.isNil            # false                 (first, export `f` in `FileStreamObj`)
import sequtils
echo s.f.isNil            # true           


for d in [1, 2, 3].mapIt($it):
  s.write d

defer is executing when it encounters import

[Araq]

Why is a toplevel defer statement even allowed? The compiler used to forbid that.

[krux02]

I guess somebody "fixed" that limitation.

[nc-x]

The code is still there

Nim/compiler/sem.nim

Lines 558 to 560 in be0a4d1

	proc semStmtAndGenerateGenerics(c: PContext, n: PNode): PNode =
	if n.kind == nkDefer:
	localError(c.config, n.info, "defer statement not supported at top level")