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+ <title>The GHC Commentary - Template Haskell</title>
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+ <h1>The GHC Commentary - Template Haskell</h1>
+ <p>
+ The Template Haskell (TH) extension to GHC adds a meta-programming
+ facility in which all meta-level code is executed at compile time. The
+ design of this extension is detailed in "Template Meta-programming for
+ Haskell", Tim Sheard and Simon Peyton Jones, <a
+ href="http://portal.acm.org/toc.cfm?id=581690&type=proceeding&coll=portal&dl=ACM&part=series&WantType=proceedings&idx=unknown&title=unknown">ACM
+ SIGPLAN 2002 Haskell Workshop,</a> 2002. However, some of the details
+ changed after the paper was published.
+ </p>
+
+ <h2>Meta Sugar</h2>
+ <p>
+ The extra syntax of TH (quasi-quote brackets, splices, and reification)
+ is handled in the module <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/deSugar/DsMeta.hs"><code>DsMeta</code></a>.
+ In particular, the function <code>dsBracket</code> desugars the four
+ types of quasi-quote brackets (<code>[|...|]</code>,
+ <code>[p|...|]</code>, <code>[d|...|]</code>, and <code>[t|...|]</code>)
+ and <code>dsReify</code> desugars the three types of reification
+ operations (<code>reifyType</code>, <code>reifyDecl</code>, and
+ <code>reifyFixity</code>).
+ </p>
+
+ <h3>Desugaring of Quasi-Quote Brackets</h3>
+ <p>
+ A term in quasi-quote brackets needs to be translated into Core code
+ that, when executed, yields a <em>representation</em> of that term in
+ the form of the abstract syntax trees defined in <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/libraries/template-haskell/Language/Haskell/TH/Syntax.hs"><code>Language.Haskell.TH.Syntax</code></a>.
+ Within <code>DsMeta</code>, this is achieved by four functions
+ corresponding to the four types of quasi-quote brackets:
+ <code>repE</code> (for <code>[|...|]</code>), <code>repP</code> (for
+ <code>[p|...|]</code>), <code>repTy</code> (for <code>[t|...|]</code>),
+ and <code>repTopDs</code> (for <code>[d|...|]</code>). All four of
+ these functions receive as an argument the GHC-internal Haskell AST of
+ the syntactic form that they quote (i.e., arguments of type <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/hsSyn/HsExpr.lhs"><code>HsExpr</code></a><code>.HsExpr
+ Name</code>, <a href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/hsSyn/HsPat.lhs"><code>HsPat</code></a><code>.HsPat Name</code>,
+ <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/hsSyn/HsTypes.lhs"><code>HsType</code></a><code>.HsType
+ Name</code>, and <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/hsSyn/HsDecls.lhs"><code>HsDecls</code></a><code>.HsGroup
+ Name</code>, respectively).
+ </p>
+ <p>
+ To increase the static type safety in <code>DsMeta</code>, the functions
+ constructing representations do not just return plain values of type <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/coreSyn/CoreSyn.lhs"><code>CoreSyn</code></a>
+ <code>.CoreExpr</code>; instead, <code>DsMeta</code> introduces a
+ parametrised type <code>Core</code> whose dummy type parameter indicates
+ the source-level type of the value computed by the corresponding Core
+ expression. All construction of Core fragments in <code>DsMeta</code>
+ is performed by smart constructors whose type signatures use the dummy
+ type parameter to constrain the contexts in which they are applicable.
+ For example, a function that builds a Core expression that evaluates to
+ a TH type representation, which has type
+ <code>Language.Haskell.TH.Syntax.Type</code>, would return a value of
+ type
+ </p>
+ <blockquote>
+ <pre>
+Core Language.Haskell.TH.Syntax.Type</pre>
+ </blockquote>
+
+ <h3>Desugaring of Reification Operators</h3>
+ <p>
+ The TH paper introduces four reification operators:
+ <code>reifyType</code>, <code>reifyDecl</code>,
+ <code>reifyFixity</code>, and <code>reifyLocn</code>. Of these,
+ currently (= 9 Nov 2002), only the former two are implemented.
+ </p>
+ <p>
+ The operator <code>reifyType</code> receives the name of a function or
+ data constructor as its argument and yields a representation of this
+ entity's type in the form of a value of type
+ <code>TH.Syntax.Type</code>. Similarly, <code>reifyDecl</code> receives
+ the name of a type and yields a representation of the type's declaration
+ as a value of type <code>TH.Syntax.Decl</code>. The name of the reified
+ entity is mapped to the GHC-internal representation of the entity by
+ using the function <code>lookupOcc</code> on the name.
+ </p>
+
+ <h3>Representing Binding Forms</h3>
+ <p>
+ Care needs to be taken when constructing TH representations of Haskell
+ terms that include binding forms, such as lambda abstractions or let
+ bindings. To avoid name clashes, fresh names need to be generated for
+ all defined identifiers. This is achieved via the routine
+ <code>DsMeta.mkGenSym</code>, which, given a <code>Name</code>, produces
+ a <code>Name</code> / <code>Id</code> pair (of type
+ <code>GenSymBind</code>) that associates the given <code>Name</code>
+ with a Core identifier that at runtime will be bound to a string that
+ contains the fresh name. Notice the two-level nature of this
+ arrangement. It is necessary, as the Core code that constructs the
+ Haskell term representation may be executed multiple types at runtime
+ and it must be ensured that different names are generated in each run.
+ </p>
+ <p>
+ Such fresh bindings need to be entered into the meta environment (of
+ type <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/deSugar/DsMonad.lhs"><code>DsMonad</code></a><code>.DsMetaEnv</code>),
+ which is part of the state (of type <code>DsMonad.DsEnv</code>)
+ maintained in the desugarer monad (of type <code>DsMonad.DsM</code>).
+ This is done using the function <code>DsMeta.addBinds</code>, which
+ extends the current environment by a list of <code>GenSymBind</code>s
+ and executes a subcomputation in this extended environment. Names can
+ be looked up in the meta environment by way of the functions
+ <code>DsMeta.lookupOcc</code> and <code>DsMeta.lookupBinder</code>; more
+ details about the difference between these two functions can be found in
+ the next subsection.
+ </p>
+ <p>
+ NB: <code>DsMeta</code> uses <code>mkGenSym</code> only when
+ representing terms that may be embedded into a context where names can
+ be shadowed. For example, a lambda abstraction embedded into an
+ expression can potentially shadow names defined in the context it is
+ being embedded into. In contrast, this can never be the case for
+ top-level declarations, such as data type declarations; hence, the type
+ variables that a parametric data type declaration abstracts over are not
+ being gensym'ed. As a result, variables in defining positions are
+ handled differently depending on the syntactic construct in which they
+ appear.
+ </p>
+
+ <h3>Binders Versus Occurences</h3>
+ <p>
+ Name lookups in the meta environment of the desugarer use two functions
+ with slightly different behaviour, namely <code>DsMeta.lookupOcc</code>
+ and <code>lookupBinder</code>. The module <code>DsMeta</code> contains
+ the following explanation as to the difference of these functions:
+ </p>
+ <blockquote>
+ <pre>
+When we desugar [d| data T = MkT |]
+we want to get
+ Data "T" [] [Con "MkT" []] []
+and *not*
+ Data "Foo:T" [] [Con "Foo:MkT" []] []
+That is, the new data decl should fit into whatever new module it is
+asked to fit in. We do *not* clone, though; no need for this:
+ Data "T79" ....
+
+But if we see this:
+ data T = MkT
+ foo = reifyDecl T
+
+then we must desugar to
+ foo = Data "Foo:T" [] [Con "Foo:MkT" []] []
+
+So in repTopDs we bring the binders into scope with mkGenSyms and addBinds,
+but in dsReify we do not. And we use lookupOcc, rather than lookupBinder
+in repTyClD and repC.</pre>
+ </blockquote>
+ <p>
+ This implies that <code>lookupOcc</code>, when it does not find the name
+ in the meta environment, uses the function <code>DsMeta.globalVar</code>
+ to construct the <em>original name</em> of the entity (cf. the TH paper
+ for more details regarding original names). This name uniquely
+ identifies the entity in the whole program and is in scope
+ <em>independent</em> of whether the user name of the same entity is in
+ scope or not (i.e., it may be defined in a different module without
+ being explicitly imported) and has the form <module>:<name>.
+ <strong>NB:</strong> Incidentally, the current implementation of this
+ mechanisms facilitates breaking any abstraction barrier.
+ </p>
+
+ <h3>Known-key Names for Template Haskell</h3>
+ <p>
+ During the construction of representations, the desugarer needs to use a
+ large number of functions defined in the library
+ <code>Language.Haskell.TH.Syntax</code>. The names of these functions
+ need to be made available to the compiler in the way outlined <a
+ href="../the-beast/prelude.html">Primitives and the Prelude.</a>
+ Unfortunately, any change to <a
+ href="http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/ghc/compiler/prelude/PrelNames.lhs"><code>PrelNames</code></a>
+ triggers a significant amount of recompilation. Hence, the names needed
+ for TH are defined in <code>DsMeta</code> instead (at the end of the
+ module). All library functions needed by TH are contained in the name
+ set <code>DsMeta.templateHaskellNames</code>.
+ </p>
+
+ <p><small>
+<!-- hhmts start -->
+Last modified: Wed Nov 13 18:01:48 EST 2002
+<!-- hhmts end -->
+ </small>
+ </body>
+</html>
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