a [Char] is now printed as string, like in Haskell.

forkIO and forkOS are different now. The former submits a task in a fixed size thread pool executor which is provided by the runtime. The latter, as before, creates an OS thread.

In IDE mode, compiler will replace unresolved variables with undefined, and will continue working, up to and including type check. This makes it probable that unrelated code (and in many cases also code near the error, as undefined shouldn't introduce type errors) can still get typechecked, and IDE features like content assist will be able to do better work.

Syntactic sugar: _.foobar desugars to (\it -> it.fooBar). This will typecheck if and only if: the argument type of the lambda is not a type variable and the type of the argument has a foobar function or foobar is a class operation. A successful type check will rewrite the lambda like (\it -> T.fooBar it), where T is the type constructor of the type of it. Later, a simplifying compiler pass will un-eta-expand the lambda, and the result is just T.fooBar. This way, we can TDNR a function even if we have no actual syntactic argument it is applied to! This comes in handy in point free or monadic context:

letters = packed . filter _.isLetter . unpacked
foolines sss = openReader sss >>= _.getLines >>= return .  any (flip _.startsWith "foo")

The first example filters the letters of a string. The second one tells if a named file contains lines that start with "foo". Apart form saving a few keystrokes, the _.foobar notation is immune against changes of the type name. Also, you do not even have to be able to name the type taht contains foobar.

Access to characters in strings doesn't work anymore with syntax str.[index]. Use charAt. The syntactic sugar is only valid for arrays.