Implement basic bar ref ${!a}.

Polish var-ref tests.

Addresses issue #26.

core/word_eval.py

@@ -434,8 +434,11 @@ def _ApplyPrefixOp(self, val, op_id):
         # TODO: There can be empty placeholder values in the array.
         length = len(val.strs)
       return runtime.Str(str(length))
+    elif op_id == Id.VSub_Bang:
+      # Treat the value of the variable as a variable name.
+      return self.mem.GetVar(val.s)
     else:
-      raise NotImplementedError(op_id)
+      raise AssertionError(op_id)
 
   def _ApplyOtherSuffixOp(self, val, op):
 

spec/var-ref.test.sh

@@ -4,6 +4,30 @@
 #
 # http://stackoverflow.com/questions/16461656/bash-how-to-pass-array-as-an-argument-to-a-function
 
+### var ref ${!a}
+a=b
+b=c
+echo ref ${!a} ${a}
+# Woah mksh has a completely different behavior -- var name, not var ref.
+# stdout: ref c b
+# BUG mksh stdout: ref a b
+# N-I dash/zsh stdout-json: ""
+
+### declare -n and ${!a}
+declare -n a
+a=b
+b=c
+echo ${!a} ${a}
+# stdout: b c
+# N-I mksh stdout: a b
+
+### Bad var ref with ${!a}
+#set -o nounset
+a='bad var name'
+echo ref ${!a}
+echo status=$?
+# stdout-json: "ref\nstatus=0\n"
+# BUG mksh stdout-json: "ref a\nstatus=0\n"
 ### pass array by reference
 show_value() {
   local -n array=$1
@@ -42,20 +66,3 @@ caller
 # BUG mksh stdout-json: ""
 # BUG mksh status: 1
 
-### Var ref with ${!a}
-a=b
-b=c
-echo ref ${!a}
-# Woah mksh has a completely different behavior -- var name, not var ref.
-# stdout: ref c
-# BUG mksh stdout: ref a
-# N-I dash/zsh stdout-json: ""
-
-### Bad var ref with ${!a}
-#set -o nounset
-a='bad var name'
-echo ref ${!a}
-# Woah even dash implements this!
-# stdout-json: "ref\n"
-# BUG mksh stdout: ref a
-# N-I dash/zsh stdout-json: ""

test/spec.sh

@@ -452,7 +452,8 @@ extended-glob() {
 
 # ${!var} syntax -- oil should replace this with associative arrays.
 var-ref() {
-  sh-spec spec/var-ref.test.sh $BASH $MKSH "$@"
+  sh-spec spec/var-ref.test.sh --osh-failures-allowed 4 \
+    $BASH $MKSH $OSH "$@"
 }
 
 let() {