Add fragment warning code from Apollo client.

Also normalize document strings (for whitespace anyway).

Note that we are simply checking fragment strings for equality, not ensuring that fragment definitions within the larger graphql document have exact equality.

We could do the above, but it'd be quite a bit more complicated, and I'm not sure if there's much benefit. AFAICT the main reason for wanting that before was exactly this functionality.

index.js

@@ -1,6 +1,62 @@
 var parse = require('./parser').parse;
 
-var cache = {};
+// Strip insignificant whitespace
+// Note that this could do a lot more, such as reorder fields etc.
+function normalize(string) {
+  return string.replace(/[\s,]+/g, ' ').trim();
+}
+
+// A map docString -> graphql document
+var docCache = {};
+
+// A map fragmentName -> [normalized source]
+var fragmentSourceMap = {};
+
+function cacheKeyFromLoc(loc) {
+  return normalize(loc.source.body.substring(loc.start, loc.end));
+}
+
+// For testing.
+function resetCaches() {
+  docCache = {};
+  fragmentSourceMap = {};
+}
+
+// Take a unstripped parsed document (query/mutation or even fragment), and
+// check all fragment definitions, checking for name->source uniqueness
+var printFragmentWarnings = true;
+function checkFragments(ast) {
+  for (var i = 0; i < ast.definitions.length; i++) {
+    var fragmentDefinition = ast.definitions[i];
+    if (fragmentDefinition.kind === 'FragmentDefinition') {
+      var fragmentName = fragmentDefinition.name.value;
+      var sourceKey = cacheKeyFromLoc(fragmentDefinition.loc);
+
+      // We know something about this fragment
+      if (fragmentSourceMap.hasOwnProperty(fragmentName) &&
+            !fragmentSourceMap[fragmentName][sourceKey]) {
+
+        // this is a problem because the app developer is trying to register another fragment with
+        // the same name as one previously registered. So, we tell them about it.
+        if (printFragmentWarnings) {
+          console.warn("Warning: fragment with name " + fragmentName + " already exists.\n"
+            + "graphql-tag enforces all fragment names across your application to be unique; read more about\n"
+            + "this in the docs: http://dev.apollodata.com/core/fragments.html#unique-names");
+        }
+
+        fragmentSourceMap[fragmentName][sourceKey] = true;
+
+      } else if (!fragmentSourceMap.hasOwnProperty(fragmentName)) {
+        fragmentSourceMap[fragmentName] = {};
+        fragmentSourceMap[fragmentName][sourceKey] = true;
+      }
+    }
+  }
+}
+
+function disableFragmentWarnings() {
+  printFragmentWarnings = false;
+}
 
 function stripLoc (doc, removeLocAtThisLevel) {
   var docType = Object.prototype.toString.call(doc);
@@ -39,19 +95,22 @@ function stripLoc (doc, removeLocAtThisLevel) {
 }
 
 function parseDocument(doc) {
-  if (cache[doc]) {
-    return cache[doc];
+  var cacheKey = normalize(doc);
+
+  if (docCache[cacheKey]) {
+    return docCache[cacheKey];
   }
 
   var parsed = parse(doc);
-
   if (!parsed || parsed.kind !== 'Document') {
     throw new Error('Not a valid GraphQL document.');
   }
 
+  // check that all "new" fragments inside the documents are consistent with
+  // existing fragments of the same name
+  checkFragments(parsed);
   parsed = stripLoc(parsed, false);
-
-  cache[doc] = parsed;
+  docCache[cacheKey] = parsed;
 
   return parsed;
 }
@@ -80,5 +139,7 @@ function gql(/* arguments */) {
 
 // Support typescript, which isn't as nice as Babel about default exports
 gql.default = gql;
+gql.resetCaches = resetCaches;
+gql.disableFragmentWarnings = disableFragmentWarnings;
 
 module.exports = gql;