Add support for unused_exceptions

#11

coffeelint.json

@@ -128,7 +128,8 @@
         "hoist_local": false,
         "hoist_parent": true,
         "unused_variables": true,
-        "unused_arguments": false
+        "unused_arguments": false,
+        "unused_exceptions": ["_.+"]
     },
     "prefer_double_quotes": {
         "module": "coffeelint-prefer-double-quotes",

src/Scope.coffee

@@ -141,10 +141,14 @@ module.exports = class Scope
                     @options["unused_variables"]) or \
                     type is "Class" and @options["unused_classes"] or \
                     type is "Argument" and @options["unused_arguments"]
-            then do ->
+            then do =>
                 if reads.length or innerReads.length
                     return  # variable was read at least once
 
+                for exception in @options["unused_exceptions"] or []
+                    if (new RegExp("^#{exception}$")).test(name)
+                        return  # variable is allowed to be unused
+
                 for {locationData}, index in writes.concat(innerWrites)
                     # issue a variable-is-assigned-but-never-read warning every
                     # time it is accessed (here and in all child scopes)

src/index.coffee

@@ -117,7 +117,7 @@ module.exports = class Coffeescope2
                 <dt><code>shadow_exceptions</code></dt>
                 <dd>A list of regular expressions that further customizes the
                     behavior of <code>shadow</code> by allowing one or more
-                    names to be extempt from shadowing warnings. The default
+                    names to be exempt from shadowing warnings. The default
                     value is <samp>["err", "next"]</samp> to allow nesting of
                     Node.JS-style continuations. To be skipped, the name must
                     match the entire expression:
@@ -184,6 +184,33 @@ module.exports = class Coffeescope2
                     this warning. Defaults to <samp>true</samp> because of
                     historical reasons and the low rate of false positives
                     generated on most codebases.</dd>
+
+                <dt><code>unused_exceptions</code></dt>
+                <dd>A list of regular expressions that further customizes the
+                    behavior of <code>unused_</code> by allowing one or more
+                    names to be exempt from unused warnings. The default value
+                    is <samp>["_.+"]</samp> to skip names starting with
+                    underscores. To be skipped, the name must match the entire
+                    expression:
+                    <ul>
+                        <li>
+                            <samp>"ba."</samp>
+                            will match
+                                <code>"bar"</code> and
+                                <code>"baz"</code>
+                            but not
+                                <code>"bard"</code> or
+                                <code>"foobar"</code>.
+                        </li>
+                        <li>
+                            <samp>"ba.*"</samp>
+                            will match
+                                <code>"ba"</code> and
+                                <code>"bar"</code> and
+                                <code>"bard"</code>.
+                        </li>
+                    </ul>
+                </dd>
             </dl>
         """
         level: "warn"
@@ -198,7 +225,7 @@ module.exports = class Coffeescope2
 
         shadow: true  # warn when overwriting a variable from outer scope
         shadow_builtins: false  # don't warn when "assigning to" a superglobal
-        shadow_exceptions: ["err", "next"]  # list of args that may be shadowed
+        shadow_exceptions: ["err", "next"] # list of args that may be shadowed
 
         undefined: true  # warn when accessing an undefined variable
         hoist_local: true  # allow same-scope hoisting
@@ -207,6 +234,7 @@ module.exports = class Coffeescope2
         unused_variables: true  # warn when a variable is not accessed
         unused_arguments: false  # warn when an argument is not accessed
         unused_classes: true  # warn when a class is not instantiated or copied
+        unused_exceptions: ["_.+"] # list of names that can be unused
 
     lintAST: (root, {config, createError}) ->
         for spec in ScopeLinter.default().lint(root, config[@rule.name])

test/ScopeLinter/unused.coffee

@@ -424,3 +424,30 @@ describe "ScopeLinter/unused", ->
             unused_variables: true
             unused_classes: true
         }).should.have.length(0)
+
+    it "allows exceptions when instructed", ->
+        ScopeLinter.default().lint(nodes(
+            """
+            _foo = "bar"
+            class _UnusedKlass
+              method: (_arg) ->
+            """
+        ), {
+            unused_variables: true
+            unused_arguments: true
+            unused_classes: true
+            unused_exceptions: ["_.+"]
+        }).should.have.length(0)
+
+        ScopeLinter.default().lint(nodes(
+            """
+            _foo = "bar"
+            class _UnusedKlass
+              method: (_arg) ->
+            """
+        ), {
+            unused_variables: true
+            unused_arguments: true
+            unused_classes: true
+            unused_exceptions: ["_..."]
+        }).should.have.length(1) # only catches the unused class