Matthew Pickering authored 5 years ago and Marge Bot committed 5 years ago

This patch implements overloaded quotation brackets which generalise the
desugaring of all quotation forms in terms of a new minimal interface.

The main change is that a quotation, for example, [e| 5 |], will now
have type `Quote m => m Exp` rather than `Q Exp`. The `Quote` typeclass
contains a single method for generating new names which is used when
desugaring binding structures.

The return type of functions from the `Lift` type class, `lift` and `liftTyped` have
been restricted to `forall m . Quote m => m Exp` rather than returning a
result in a Q monad.

More details about the feature can be read in the GHC proposal.

https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0246-overloaded-bracket.rst

Summary:
This commit adds stage 1 support for Template Haskell
quoting, e.g. [| ... expr ... |], which is useful
for authors of quasiquoter libraries that do not actually
need splices.  The TemplateHaskell extension now does not
unconditionally fail; it only fails if the renamer encounters
a splice that it can't run.

In order to make sure the referenced data structures
are consistent, template-haskell is now a boot library.
There are some minor BC changes to template-haskell to make it boot
on GHC 7.8.

Note for reviewer: big diff changes are simply code
being moved out of an ifdef; there was no other substantive
change to that code.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin, goldfire

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D876

GHC Trac Issues: #10382

Revert "Quick fix: drop base bound on template-haskell."

This reverts commit 3c70ae03.

Revert "Always do polymorphic typed quote check, c.f. #10384"

This reverts commit 9a43b2c1.

Revert "RnSplice's staging test should be applied for quotes in stage1."

This reverts commit eb0ed403.

Revert "Split off quotes/ from th/ for tests that can be done on stage1 compiler."

This reverts commit 21c72e7d.

Revert "Support stage 1 Template Haskell (non-quasi) quotes, fixes #10382."

This reverts commit 28257cae.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: run these tests with stage1

Reviewers: simonpj, austin

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D877

GHC Trac Issues: #10382

------------------------------------
	Major increment for Template Haskell
	------------------------------------

1.  New abstract data type "Name" which appears where String used to be.
    E.g. 	data Exp = VarE Name | ...

2.  New syntax 'x and ''T, for quoting Names.  It's rather like [| x |]
    and [t| T |] respectively, except that

	a) it's non-monadic:  'x :: Name
	b) you get a Name not an Exp or Type

3.  reify is an ordinary function
	reify :: Name -> Q Info
    New data type Info which tells what TH knows about Name

4.  Local variables work properly.  So this works now (crashed before):
	f x = $( [| x |] )

5.  THSyntax is split up into three modules:

  Language.Haskell.TH		TH "clients" import this

  Language.Haskell.TH.THSyntax	data type declarations and internal stuff

  Language.Haskell.TH.THLib	Support library code (all re-exported
				by TH), including smart constructors and
				pretty printer

6.  Error reporting and recovery are in (not yet well tested)

	report :: Bool {- True <=> fatal -} -> String -> Q ()
	recover :: Q a -> Q a -> Q a

7.  Can find current module

	currentModule :: Q String


Much other cleaning up, needless to say.

Two more TH tests, thanks to Sean