Three improvements to Template Haskell (fixes #3467)
This patch implements three significant improvements to Template Haskell.
Declaration-level splices with no "$"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This change simply allows you to omit the "$(...)" wrapper for
declaration-level TH splices. An expression all by itself is
not legal, so we now treat it as a TH splice. Thus you can now
say
data T = T1 | T2
deriveMyStuff ''T
where deriveMyStuff :: Name -> Q [Dec]
This makes a much nicer interface for clients of libraries that use
TH: no scary $(deriveMyStuff ''T).
Nested top-level splices
~~~~~~~~~~~~~~~~~~~~~~~~
Previously TH would reject this, saying that splices cannot be nested:
f x = $(g $(h 'x))
But there is no reason for this not to work. First $(h 'x) is run,
yielding code <blah> that is spliced instead of the $(h 'x). Then (g
<blah>) is typechecked and run, yielding code that replaces the
$(g ...) splice.
So this simply lifts the restriction.
Fix Trac #3467: non-top-level type splices
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It appears that when I added the ability to splice types in TH
programs, I failed to pay attention to non-top-level splices -- that
is, splices inside quotatation brackets.
This patch fixes the problem. I had to modify HsType, so there's a
knock-on change to Haddock.
Its seems that a lot of lines of code has changed, but almost all the
new lines are comments!
General tidying up
~~~~~~~~~~~~~~~~~~
As a result of thinking all this out I re-jigged the data type ThStage,
which had far too many values before. And I wrote a nice state transition
diagram to make it all precise;
see Note [Template Haskell state diagram] in TcSplice
Lots more refactoring in TcSplice, resulting in significantly less code.
(A few more lines, but actually less code -- the rest is comments.)
I think the result is significantly cleaner.
15 files changed: