Implement C-style for loops.

Addresses issue #66.

Also:
- Improve the error message when a construct isn't implemented.

core/cmd_exec.py

@@ -971,7 +971,15 @@ def _Dispatch(self, node, fork_external):
        self.loop_level -= 1

    elif node.tag == command_e.ForExpr:
      raise NotImplementedError(node.tag)
      status = 0
      self.arith_ev.Eval(node.init)
      while True:
        b = self.arith_ev.Eval(node.cond)
        if not b:
          break

        status = self._Execute(node.body)
        self.arith_ev.Eval(node.update)

    elif node.tag == command_e.DoGroup:
      status = self._ExecuteList(node.children)
@@ -1045,7 +1053,7 @@ def _Dispatch(self, node, fork_external):
      print('sys\t%.3f' % sys_, file=sys.stderr)

    else:
      raise AssertionError(node.tag)
      raise NotImplementedError(node.__class__.__name__)

    return status, check_errexit

spec/for-expr.test.sh

@@ -9,10 +9,15 @@ for ((a=1; a <= n ; a++))  # Double parentheses, and naked 'n'
do
  echo $a
done  # A construct borrowed from ksh93.
# status: 0
# stdout-json: "1\n2\n3\n4\n5\n"
# N-I mksh status: 1
# N-I mksh stdout-json: ""
## status: 0
## STDOUT:
1
2
3
4
5
## N-I mksh status: 1
## N-I mksh stdout-json: ""

### For loop with and without semicolon
for ((a=1; a <= 3; a++)); do
@@ -21,8 +26,13 @@ done  # A construct borrowed from ksh93.
for ((a=1; a <= 3; a++)) do
  echo $a
done  # A construct borrowed from ksh93.
# status: 0
# stdout-json: "1\n2\n3\n1\n2\n3\n"
# N-I mksh status: 1
# N-I mksh stdout-json: ""

## status: 0
## STDOUT:
1
2
3
1
2
3
## N-I mksh status: 1
## N-I mksh stdout-json: ""

test/spec.sh

@@ -529,7 +529,7 @@ let() {
}

for-expr() {
  sh-spec spec/for-expr.test.sh --osh-failures-allowed 2 \
  sh-spec spec/for-expr.test.sh \
    $MKSH $BASH $OSH "$@"
}