Changeset 190

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• docs/trunk/hacking-guide/tock-intro.tex

  r189	r190
  404	404	\subsection{Generics}
  405	405
  406	%TODO
  406	We use the \lstinline|Data.Generics| module of GHC to do our generics (also known as the Scrap Your
  407	Boilerplate or SYB approach). The deep-down mechanics are very confusing. How to use it is simpler,
  408	and is explained in the three excellent SYB papers. Things are made slightly tricky again because
  409	we usually perform custom traversals.
  410
  411	The `everywhere' (or `everywhereM') function(s) described in the SYB papers traverse an entire tree
  412	structure looking to apply your transformation. Unfortunately this includes examining each character
  413	of each string in every Meta tag, which makes things (unacceptably) slow. Therefore Adam implemented
  414	a custom traversal pattern. Since you will almost always want this traversal, you do not have to
  415	worry too much about the internals, just about how to use it.
  416
  417	Here's an example; the wrapper from our previous code example:
  418
  419	\begin{lstlisting}
  420	removeParAssign :: Data t => t -> PassM t
  421	removeParAssign = doGeneric `extM` doProcess
  422	where
  423	doGeneric :: Data t => t -> PassM t
  424	doGeneric = makeGeneric removeParAssign
  425
  426	doProcess :: A.Process -> PassM A.Process
  427	doProcess (A.Assign m vs@(_:_:_) (A.ExpressionList _ es)) = ...
  428	doProcess p = doGeneric p
  429	\end{lstlisting}
  430
  431	You should follow this template for any new passes you write. All you need to customise is the
  432	name of the pass (of course), and change the type and name of the doProcess function if you
  433	want to process something other than a process. For example:
  434
  435	\begin{lstlisting}
  436	twiddleExpressions :: Data t => t -> PassM t
  437	twiddleExpressions = doGeneric `extM` doExpression
  438	where
  439	doGeneric :: Data t => t -> PassM t
  440	doGeneric = makeGeneric twiddleExpressions
  441
  442	doExpression :: A.Expression -> PassM A.Expression
  443	doExpression (...) = ... -- First pattern match
  444	doExpression (...) = ... -- Second pattern match
  445	doExpression p = doGeneric p
  446	\end{lstlisting}
  447
  448	Note that you must include the last case for your doProcess/doExpression function; otherwise you
  449	will get an error if your pattern-matches are not exhaustive (which they rarely will be).
  450	The net effect is to apply doExpression to all expressions in the given AST, in an efficient manner.
  451	In other words, it's a compiler pass that operates on the expressions in the tree.
  407	452
  408	453	\section{Add Your Own Code}

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