Add library for simple logging, specifically for debugging.

changelog.txt

@@ -13,7 +13,8 @@ Breaking Changes:
 
 Improvements:
 
-  * Support Trampolining (thanks to @maxsnew and @timthelion) 
+  * Add Trampoline library (thanks to @maxsnew and @timthelion) 
+  * Add Debug library (inspired by @timthelion)
   * Drastically improved performance on markdown parsing (thanks to @Dandandan) 
   * Add Date.fromTime function
   * Use pointer-events to detect hovers on layered elements (thanks to @Xashili)

libraries/Debug.elm

@@ -0,0 +1,17 @@
+module Debug where
+{-| This library is for investigating bugs or performance problems. It should
+*not* be used in production code.
+-}
+
+import Native.Debug
+
+{-| Log a tagged value on the developer console, and then return the value.
+
+      1 + log "number" 1        -- equals 2, logs "number: 1"
+      length (log "start" [])   -- equals 0, logs "start: []"
+
+Notice that `log` is not a pure function! It should *only* be used for
+investigating bugs or performance problems.
+-}
+log : String -> a -> a
+log = Native.Debug.log

libraries/Native/Debug.js

@@ -0,0 +1,22 @@
+Elm.Native.Debug = {};
+Elm.Native.Debug.make = function(elm) {
+    elm.Native = elm.Native || {};
+    elm.Native.Debug = elm.Native.Debug || {};
+    if (elm.Native.Debug.values) return elm.Native.Debug.values;
+
+    var show = Elm.Native.Show.make(elm).show;
+
+    function log(tag,value) {
+        if (process && process.stdout) {
+            process.stdout.write(tag + ': ' + show(value));
+        } else {
+            console.log(tag + ': ' + show(value));
+        }
+        return value;
+    }
+
+    return elm.Native.Debug.values = {
+        log: F2(log)
+    };
+    
+};