Merge pull request #488 from felixmulder/topic/post-dotty-errors

Add Dotty error messages post

Author: heathermiller

blog/_posts/2016-10-14-dotty-errors.md

@@ -0,0 +1,104 @@
+---
+layout: blog
+post-type: blog
+by: Felix Mulder
+title: "Awesome Error Messages for Dotty"
+---
+
+One thing that really excites me about being part of the core group of
+developers working on [Dotty](http://lampepfl.github.io/dotty) is my chance to
+impact usability. A lot of thought has gone into designing Dotty to be as fast
+and structurally sound as possible.  Now comes the next step - adding a new
+level of usability for the compiler and the surrounding tools.
+
+We've looked at how other modern languages like
+[Elm](http://elm-lang.org/blog/compiler-errors-for-humans) and
+[Rust](https://blog.rust-lang.org/2016/08/10/Shape-of-errors-to-come.html)
+handle compiler warnings and error messages, and come to realize that Dotty is
+actually in great shape to provide comprehensive and easy to understand error
+messages in the same spirit.
+
+Let's dive right into some examples, let's say you have this code:
+
+    try {
+      foo()
+    }
+
+It doesn't really make sense to put this in a `try`-block for two reasons:
+
+1. It doesn't throw an exception
+2. It doesn't have a `catch` or `finally` clause
+
+So let's say we compile this file using scalac, we get something like:
+
+<pre>
+test.scala:2: warning: A try without a catch or finally is equivalent to putting its body in a block; no exceptions are handled.
+     try {
+       ^
+       one warning found
+</pre>
+
+This is helpful, but it has a couple of drawbacks:
+
+1. If we have a bazillion errors, it will be hard to read
+2. If we don't know about `catch` or `finally` blocks - we don't know how to
+   solve this (yes I know, most people do know what these are but - toy
+   example!)
+
+So what do you get with Dotty? This:
+
+![try-error](http://i.imgur.com/vNE706E.png)
+
+All errors are now visually separated, and the output is colorized so that you
+can find your mistakes quickly.
+
+Another one of our goals is to be able to properly explain things when asked.
+As such, if you pass the flag `-explain` when compiling the example above,
+you'll get a more verbose explanation:
+
+![try-error2](http://i.imgur.com/pNhgsdf.png)
+
+Mistyping members
+-----------------
+Sometimes, especially when you're in a rush - you might mistype some members.
+Currently we offer you the following support when selecting on a type:
+
+    class Foo {
+      def bar = ???
+    }
+
+    val foo = new Foo()
+    foo.barr
+
+Will yield:
+
+![mistype](http://i.imgur.com/iDnpB9O.png)
+
+In the future we want to be able to offer you these types of suggestions on
+other things like missing imports.
+
+Type diffs
+----------
+Sometimes when working with complex types - it's hard to see exactly where the
+error occurs. The Dotty compiler will in these cases give you a colored diff:
+
+![mismatch](http://i.imgur.com/vlmwrmD.png)
+
+It will not do this however if the differences are huge - but it will syntax
+highlight the found and expected type anyway.
+
+We want you!
+------------
+To make the transition to these new error messages as quick and pain-free as
+possible - we need help! This is a perfect entry-point into hacking on the
+compiler as you'll need to create semantic objects that contain the relevant
+information for the error or warning.
+
+So - this is what you do:
+
+1. Go to the [Error messages issue](https://github.com/lampepfl/dotty/issues/1589)
+2. Read the howto on error messages
+3. Choose an error message you want to help with and post a comment saying
+   which one(s)
+4. Get hacking
+5. Submit a PR!