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Implementing GHC External Core: an experiment with the BNF Converter
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The starting point of the work are the files
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ExternalCore.lhs and ParserExternalCore.y contained in the GHC External Core
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source package. The goal is to get an abstract syntax as close as possible
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to the original. The hope is that External Core is well-behaved, since
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it has few concerns for "user-friendly" syntax.
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THE PHASES OF THE WORK
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Start 11.10, by converting the abstract syntax file to BNF grammar
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with nonterminals only. Finished 11.24. Then go through the Happy
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file to insert terminals properly. At 12.00 the resulting grammar file,
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Core.cf, gets compiled in bnfc, but gets some conflicts.
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Aften an hour's lunch break, find the obvious reduce/reduce
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conflict between Var and DCon in Exp. Change Var to unqualified
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identifiers. Start testing with a hello world -program Hello.hcr,
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ghc -fext-core AbsCore.hs Hello.hs
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This gets parsed, but some other files don't. The reason turns out
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to be that GHC 5.02.2 generates qualified identifiers where the
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Core syntax expects qualified ones. Changing this does not quite suffice,
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however, since GHC also generates qualified ones; therefore,
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divide the constructor for Vdef into two, VdefQ and VdefU.
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At this point, test files get so big (3860 in AbsCore.hcr) that Hugs does
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manage them, so create a compilable test file TopCore, compiled with
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ghc --make -i/home/aarne/BNFC TopCore.hs -o TopCore
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Now manage to parse AbsCoreAt at 14.02, after 1 hour's work on grammar writing,
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1 hour on debugging. One reduce/reduce conflict remains. Come back to it
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later and locate it in the %forall rule: I had just missed the dot (".") in
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the Happy file! The ParCore.info file was useful in locating it, in 5 minutes.
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Trying other examples, there is still a problem with string and character
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literals: the standard ones of BNFC do not handle everything that appears in Core.
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First experiment with changes in the generated file LexCore.x. Then make it
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the proper way, by defining the token types Str and Chr in Core.cf.
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At 15.18 manage to parse all my examples with the BNFC-generated parser, the
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biggest one being the parser of Core itself:
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64907 143661 1797533 ParCore.hcr
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15.18 made the Str and Chr token definitions in Core.cf. Now manage to
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parse all examples completely with BNFC-generated parser.
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This document was being written as book-keeping while programming.
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Some clean-up was done afterwards, and also some comments were added
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to Core.cf. Next morning, the document was rewritten to the current shape.
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The work took 1h grammar writing, 1h debugging, 30m fine-tuning.
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The resulting grammar parses all tested examples, but the abstract
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syntax is slightly different, mostly due to BNFC not having polymorphic
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pair and Maybe types. In addition, the original uses some foldr's
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as semantic actions, where we just have to retain the lists.
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The External Core language is reasonably well-behaved, and the source
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files gave good support to the grammar development.
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The pretty-printer might be fine-tuned. In particular, the qualifier
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dots (but not the %forall dots!) should not be separated by spaces.
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It seems straightforward to translate back and forth between the
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original syntax and our AbsCore.hs. However, if the External Core
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language had been defined in the BNF converter language from the
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beginning, this would not be necessary. The generated abstract syntax
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is not very much worse then the hand-written one. There would be
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a guaranteed match between the abstract syntax, the parser, the pretty
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printer, and the language document - and only a fraction of the current
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amount of code and text would have had to be written:
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89 243 1324 ExternalCore.lhs
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240 1042 5168 ParserExternalCore.y
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168 906 4667 PprExternalCore.lhs
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where the lexer source and the language document are still missing.
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The BNF Converter: http://www.cs.chalmers.se/~markus/BNFC/
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GHC External Core: http://www.haskell.org/ghc/docs/papers/core.ps.gz
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examples/haskell-core/notes.txt
