Add throwWhenCannotReduce option

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 throwWhenCannotReduce = options.throwWhenCannotReduce
 
   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 (throwWhenCannotReduce && CONTAINS_CALC.test(value)) {
+          throw decl.error("Could not reduce expression: " + decl.value,
+            {plugin: "postcss-calc"})
+        }
+
         if (!preserve) {
           decl.value = value
           return

test/fixtures/throwWhenCannotReduce.css

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

test/index.js

@@ -55,5 +55,15 @@ test("calc", function(t) {
     "should have a preserve option that allow to keep original calc() usage"
   )
 
+  t.throws(function() {
+    var name = "throwWhenCannotReduce"
+
+    postcss()
+      .use(calc({throwWhenCannotReduce: true}))
+      .process(fixture(name), {from: fixturePath(name)})
+      .css
+  }, "should have throwWhenCannotReduce option that throws when calc() " +
+    "cannot be reduced")
+
   t.end()
 })