implement defer statement

[mattn]

need tests.

[codecov-io]

Codecov Report

Merging #195 into master will decrease coverage by 1.78%.
The diff coverage is 6.25%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
- Coverage   92.02%   90.24%   -1.79%     
==========================================
  Files           9        9              
  Lines        2270     2317      +47     
==========================================
+ Hits         2089     2091       +2     
- Misses        118      162      +44     
- Partials       63       64       +1

Impacted Files	Coverage Δ
vm/env.go	100% <ø> (ø)	⬆️
vm/vm.go	86.14% <ø> (ø)	⬆️
vm/vmStmt.go	89.52% <0%> (-7.48%)	⬇️
vm/vmExprFunction.go	75.53% <33.33%> (-1.48%)	⬇️

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[mattn]

Anko's defer statement doesn't work like Go's one since anko doesn't have named return parameters for a function. Anko can not re-write return value. Also current implementation doesn't have recover().

[mattn]

func foo() {
  var i = 0
  defer println("foo", i)
  i++
  defer println("bar", i)
  i++
  defer println("bar", i)
  i++
  defer println("bar", i)
  i++
  defer println("bar", i)
}

foo()

This works fine.

bar 4
bar 3
bar 2
bar 1
foo 0

[MichaelS11]

Could we put the AnonCallExpr and CallExpr in vmExprFunction.go then be able to use it from case *ast.DeferStmt as well as the current places:

	case *ast.AnonCallExpr:
		return anonCallExpr(e, env)

	case *ast.CallExpr:
		return callExpr(e, env)

Would be really nice if there was a way to merge the two AnonCallExpr & CallExpr so that we do not have to maintain two code areas.

[mattn]

This implementation is strange since VM run defer even if it is in if/for block. It must be in func block.

var time = import("time")
for {
  defer println("foo")
  time.Sleep(1e9)
}

foo must not be appeared.

[MichaelS11]

I still wonder if some of this code is not needed. Say they do defer something that is not a function, like a++. If that is passed to say anonCallExpr as an example, that will error and say it is not a function. I think you can just simplify syntax to just DEFER exprs.

[MichaelS11]

Was curious, is it an issue if the code can defer any expression, not just functions?

[xtlsoft]

I think maybe we need to execute defer statements when an env destroys.