Add throwWhenCannotReduce option

CHANGELOG.md

@@ -1,3 +1,5 @@
+- Added: "warnWhenCannotResolve" option to warn when calc() are not reduced to a single value
+
 # 5.0.0 - 2015-08-25
 
 - Removed: compatibility with postcss v4.x

README.md

@@ -107,6 +107,17 @@ var out = postcss()
   .css
 ```
 
+#### `warnWhenCannotResolve` (default: `false`)
+
+Adds warnings when calc() are not reduced to a single value.
+
+```js
+var out = postcss()
+  .use(calc({warnWhenCannotResolve: true}))
+  .process(css)
+  .css
+```
+
 ---
 
 ## Contributing

index.js

@@ -5,15 +5,19 @@ var reduceCSSCalc = require("reduce-css-calc")
 var helpers = require("postcss-message-helpers")
 var postcss = require("postcss")
 
+var CONTAINS_CALC = /calc\(.*\)/
+
 /**
  * PostCSS plugin to reduce calc() function calls.
  */
 module.exports = postcss.plugin("postcss-calc", function(options) {
   options = options || {}
   var precision = options.precision
   var preserve = options.preserve
+  var warnWhenCannotResolve = options.warnWhenCannotResolve
+
+  return function(style, result) {
 
-  return function(style) {
     style.walkDecls(function transformDecl(decl) {
       if (!decl.value || decl.value.indexOf("calc(") === -1) {
         return
@@ -22,6 +26,11 @@ module.exports = postcss.plugin("postcss-calc", function(options) {
       helpers.try(function transformCSSCalc() {
         var value = reduceCSSCalc(decl.value, precision)
 
+        if (warnWhenCannotResolve && CONTAINS_CALC.test(value)) {
+          result.warn("Could not reduce expression: " + decl.value,
+            {plugin: "postcss-calc", node: decl})
+        }
+
         if (!preserve) {
           decl.value = value
           return

test/fixtures/warnWhenCannotResolve.css

@@ -0,0 +1,3 @@
+calc {
+  unresolved: calc(1.125rem - 1px);
+}

test/fixtures/warnWhenCannotResolve.expected.css

@@ -0,0 +1,3 @@
+calc {
+  unresolved: calc(1.125rem - 1px);
+}

test/index.js

@@ -14,22 +14,24 @@ function fixture(name) {
 }
 
 function compareFixtures(t, name, options, message) {
-  var actual = postcss()
+  var result = postcss()
     .use(customProperties())
     .use(calc(options))
     .process(fixture(name), {from: fixturePath(name)})
-    .css.trim()
+  var actual = result.css.trim()
 
   // handy thing: checkout actual in the *.actual.css file
   fs.writeFile(fixturePath(name + ".actual"), actual)
 
-  return t.equal(
+  t.equal(
     actual,
     fixture(name + ".expected"),
     message
       ? message
       : "processed fixture '" + name + "' should be equal to expected output"
   )
+
+  return result
 }
 
 test("calc", function(t) {
@@ -55,5 +57,16 @@ test("calc", function(t) {
     "should have a preserve option that allow to keep original calc() usage"
   )
 
+  var result = compareFixtures(
+    t,
+    "warnWhenCannotResolve",
+    {warnWhenCannotResolve: true}
+  )
+
+  t.ok(
+    result.messages[0].text.match(/^Could not reduce expression:/),
+    "should add a warning for unreduced calc() "
+  )
+
   t.end()
 })