[oil-language] Implement overlooked != operator

Caught by Danilo Spinella.

- Add test cases from the other examples.
- Language FAQ: Explain limited proc / shell function return values.

Thanks for the feedback!

doc/oil-language-faq.md

@@ -16,6 +16,7 @@ Old and new constructs exist side-by-side.  New constructs have fewer
 <div id="toc">
 </div>
 
+
 ## What's the difference between `$(dirname $x)` and `$len(x)` ?
 
 Superficially, both of these syntaxes take an argument `x` and return a
@@ -29,6 +30,21 @@ string.  But they are different:
 
 (Note: command subs may be optimized later, as `ksh` does.)
 
+## How can I return rich values from shell functions / Oil `proc`s?
+
+There are two primary ways:
+
+- Print the "returned" data to `stdout`.  Retrieve it with a command sub like
+  `$(myproc)` or a pipeline like `myproc | read --line`.
+- Use an "out param" with [setref]($oil-help:setref).
+
+Oil may grow true functions with the `func` keyword at some point.  However,
+that must be done carefully, as a `proc` composes with processes, but a `func`
+doesn't.
+
+Send us feedback if this doesn't make sense, or if you want a longer
+explanation.
+
 ## Why doesn't a raw string work here: `${array[r'\']}` ?
 
 Oil has two array index syntax:

oil_lang/expr_eval.py

@@ -350,8 +350,11 @@ def EvalExpr(self, node):
           result = left >= right
         elif op.id == Id.Arith_LessEqual:
           result = left <= right
+
         elif op.id == Id.Arith_DEqual:
           result = left == right
+        elif op.id == Id.Arith_NEqual:
+          result = left != right
 
         elif op.id == Id.Expr_In:
           result = left in right

spec/oil-user-feedback.test.sh

@@ -0,0 +1,55 @@
+# Oil user feedback
+
+# From Zulip:
+#
+# https://oilshell.zulipchat.com/#narrow/stream/121540-oil-discuss/topic/Experience.20using.20oil
+
+#### setvar
+
+# This seems to work as expected?
+
+proc get_opt(arg, :out) {
+  setvar out = $arg
+}
+var a = ''
+get_opt a 'lol'
+echo hi
+## status: 1
+## STDOUT:
+## END
+
+#### != operator
+var a = 'bar'
+
+# NOTE: a != foo is idiomatic)
+if ($a != 'foo') {
+  echo 'not equal'
+}
+
+if ($a != 'bar') {
+  echo 'should not get here'
+}
+
+## STDOUT:
+not equal
+## END
+
+
+#### Regex Literal
+
+# This seems to work as expected?
+
+proc get_opt(arg, :out) {
+  setref out = $(write -- $arg | cut -d'=' -f2)
+}
+
+var variant = ''
+var arg = '--variant=foo'
+if ( arg ~ / '--variant=' <word> / ) {
+  get_opt $arg :variant
+  echo variant=$variant
+}
+
+## STDOUT:
+variant=foo
+## END

test/spec.sh

@@ -980,6 +980,11 @@ oil-interactive() {
     $OIL_LIST "$@"
 }
 
+oil-user-feedback() {
+  sh-spec spec/oil-user-feedback.test.sh --osh-failures-allowed 0 \
+    $OIL_LIST "$@"
+}
+
 ble-idioms() {
   sh-spec spec/ble-idioms.test.sh --osh-failures-allowed 0 \
           $BASH $ZSH $MKSH $BUSYBOX_ASH $OSH_LIST "$@"