@-bindings broken in 6.0?

[Simon Peyton-Jones]

|  > 	-farrows
|=20
| Is this switch documented somewhere? I looked through the docs in the
| latest CVS version of GHC and couldn't find anything about it ...

Not yet: I only put it in this week!  And it's not documented yet.
There are some examples, though
in
	fptools/testsuite/tests/ghc-regress/arrows

And the commit message contains many details (enclosed)

Simon

	----------------------------------------------
	Add support for Ross Paterson's arrow notation
	----------------------------------------------

Ross Paterson's ICFP'01 paper described syntax to support John Hughes's
"arrows", rather as do-notation supports monads.  Except that
do-notation is
relatively modest -- you can write monads by hand without much trouble
--
whereas arrow-notation is more-or-less essential for writing arrow
programs.
It desugars to a massive pile of tuple construction and selection!

For some time, Ross has had a pre-processor for arrow notation, but the
resulting type error messages (reported in terms of the desugared code)
are impenetrable.  This commit integrates the syntax into GHC.  The=20
type error messages almost certainly still require tuning, but they
should
be better than with the pre-processor.

Main syntactic changes (enabled with -farrows)
  =20
   exp ::=3D ... | proc pat -> cmd

   cmd ::=3D exp1 -<  exp2   |  exp1 >-  exp2
	|  exp1 -<< exp2   |  exp1 >>- exp2
	| \ pat1 .. patn -> cmd
	| let decls in cmd
	| if exp then cmd1 else cmd2
	| do { cstmt1 .. cstmtn ; cmd }
	| (| exp |) cmd1 .. cmdn
	| cmd1 qop cmd2
	| case exp of { calts }

   cstmt :: =3D let decls
	 |   pat <- cmd
	 |   rec { cstmt1 .. cstmtn }
	 |   cmd

New keywords and symbols:
	proc rec
	-<   >-   -<<   >>- =20
	(|  |)

The do-notation in cmds was not described in Ross's ICFP'01 paper;
instead
it's in his chapter in The Fun of Programming (Plagrave 2003).

The four arrow-tail forms (-<) etc cover=20
  (a) which order the pices come in (-<  vs  >-), and=20
  (b) whether the locally bound variables can be used in the
		arrow part (-<  vs  -<<) . =20
In previous presentations, the higher-order-ness (b) was inferred,
but it makes a big difference to the typing required so it seems more
consistent to be explicit.

The 'rec' form is also available in do-notation:
  * you can use 'rec' in an ordinary do, with the obvious meaning
  * using 'mdo' just says "infer the minimal recs"


Still to do
~~~~~~~~~~~
Top priority is the user manual.

The implementation still lacks an implementation of
the case form of cmd.


Implementation notes
~~~~~~~~~~~~~~~~~~~~
Cmds are parsed, and indeed renamed, as expressions.  The type checker=20
distinguishes the two. =20