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9 <h1>The GHC Commentary - Just Syntax</h1>
11 The lexical and syntactic analyser for Haskell programs are located in
13 href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/parser/"><code>fptools/ghc/compiler/parser/</code></a>.
18 The lexer is a rather tedious piece of Haskell code contained in the
20 href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/parser/Lex.lhs"><code>Lex</code></a>.
21 Its complexity partially stems from covering, in addition to Haskell 98,
22 also the whole range of GHC language extensions plus its ability to
23 analyse interface files in addition to normal Haskell source. The lexer
24 defines a parser monad <code>P a</code>, where <code>a</code> is the
25 type of the result expected from a successful parse. More precisely, a
28 data ParseResult a = POk PState a
29 | PFailed Message</pre>
32 is produced with <code>Message</code> being from <a
33 href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/main/ErrUtils.lhs"><code>ErrUtils</code></a>
34 (and currently is simply a synonym for <code>SDoc</code>).
36 The record type <code>PState</code> contains information such as the
37 current source location, buffer state, contexts for layout processing,
38 and whether Glasgow extensions are accepted (either due to
39 <code>-fglasgow-exts</code> or due to reading an interface file). Most
40 of the fields of <code>PState</code> store unboxed values; in fact, even
41 the flag indicating whether Glasgow extensions are enabled is
42 represented by an unboxed integer instead of by a <code>Bool</code>. My
43 (= chak's) guess is that this is to avoid having to perform a
44 <code>case</code> on a boxed value in the inner loop of the lexer.
46 The same lexer is used by the Haskell source parser, the Haskell
47 interface parser, and the package configuration parser.
49 <h2>The Haskell Source Parser</h2>
51 The parser for Haskell source files is defined in the form of a parser
52 specification for the parser generator <a
53 href="http://haskell.org/happy/">Happy</a> in the file <a
54 href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/parser/Parser.y"><code>Parser.y</code></a>.
55 The parser exports three entry points for parsing entire modules
56 (<code>parseModule</code>, individual statements
57 (<code>parseStmt</code>), and individual identifiers
58 (<code>parseIdentifier</code>), respectively. The last two are needed
59 for GHCi. All three require a parser state (of type
60 <code>PState</code>) and are invoked from <a
61 href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/main/HscMain.lhs"><code>HscMain</code></a>.
63 <h2>The Haskell Interface Parser</h2>
65 The parser for interface files is also generated by Happy from <a href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/rename/ParseIface.y"><code>ParseIface.y</code></a>.
66 It's main routine <code>parseIface</code> is invoked from <a href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/rename/RnHiFiles.lhs"><code>RnHiFiles</code></a><code>.readIface</code>.
68 <h2>The Package Configuration Parser</h2>
70 The parser for configuration files is by far the smallest of the three
71 and defined in <a href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/main/ParsePkgConf.y"><code>ParsePkgConf.y</code></a>.
72 It exports <code>loadPackageConfig</code>, which is used by <a href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/main/DriverState.hs"><code>DriverState</code></a><code>.readPackageConf</code>.
76 Last modified: Sun Nov 18 21:22:38 EST 2001