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.
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