Changeset 182 for docs/trunk/hacking-guide/tock-intro.tex

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

  r181	r182
  77	77	\subsection{A.Structured}
  78	78
  79	%TODO
  79	The main item in the AST that I (Neil) found confusing at first was the Structured item. In case it
  80	confuses anyone else I thought I would explain it here. Structured is the body of most occam constructs,
  81	such as SEQ, PAR, ALT, CASE. Because occam allows the inter-mingling of processes and declarations,
  82	and also replication on most of its constructs (SEQ, PAR, ALT, IF) Structured eliminates redundancy
  83	by grouping this together. So for example, given this occam psuedo-code:
  84
  85	\occamsettings\begin{lstlisting}
  86	SEQ
  87	proc1
  88	proc2
  89	\end{lstlisting}
  90
  91	Here is how it would be represented in the AST (Taking proc1 and proc2 to be of type Process, and using
  92	`m' for all meta-tags):
  93
  94	\haskellsettings\begin{lstlisting}
  95	A.Seq m
  96	(A.Several m
  97	[A.OnlyP m proc1
  98	,A.OnlyP m proc2
  99	]
  100	)
  101	\end{lstlisting}
  102
  103	You can see the combination of Seq with Several and OnlyP to nest the processes. Here's another example
  104	of some occam and corresponding Haskell:
  105
  106	\occamsettings\begin{lstlisting}
  107	SEQ
  108	proc1
  109	PAR
  110	proc2
  111	proc3
  112	\end{lstlisting}
  113
  114	\haskellsettings\begin{lstlisting}
  115	A.Seq m
  116	(A.Several m
  117	[A.OnlyP m proc1
  118	,A.OnlyP m
  119	(A.Par m A.PlainPar
  120	(A.Several m
  121	[A.OnlyP m proc2
  122	,A.OnlyP m proc3
  123	]
  124	)
  125	)
  126	]
  127	)
  128	\end{lstlisting}
  129
  130	Which no doubt looks quite nasty! But things work differently if you nest two blocks of the same type,
  131	mainly because of the associativity of the various blocks in occam. Consider these two SEQ blocks:
  132
  133	\occamsettings\begin{lstlisting}
  134	SEQ
  135	proc1
  136	SEQ
  137	proc2
  138	proc3
  139	\end{lstlisting}
  140
  141	\haskellsettings\begin{lstlisting}
  142	A.Seq m
  143	(A.Several m
  144	[A.OnlyP m proc1
  145	, (A.Several m
  146	[A.OnlyP m proc2
  147	,A.OnlyP m proc3
  148	]
  149	)
  150	]
  151	)
  152	\end{lstlisting}
  153
  154	You can see that instead of creating a second \lstinline|A.Seq| inside the \lstinline|A.Several|, it
  155	has instead simply nested another \lstinline|A.Several|. In fact, we could later flatten the two
  156	nested \lstinline|A.Several|s into one if we wanted.
  157
  158	Here is another example:
  159
  160	\occamsettings\begin{lstlisting}
  161	PAR
  162	proc1
  163	PAR i = 0 FOR 10
  164	proc2
  165	\end{lstlisting}
  166
  167	\haskellsettings\begin{lstlisting}
  168	A.Par m A.PlainPar
  169	(A.Several m
  170	[A.OnlyP m proc1
  171	,A.Rep m rep (A.OnlyP m proc2)
  172	]
  173	)
  174	\end{lstlisting}
  175
  176	I have used `rep' as a short-hand for the replicator (which is not the focus here). Hopefully it is
  177	now clear how Structured is used as a body for things. There is only one more aspect to explain,
  178	which is definitions.
  179
  180	%TODO explain definitions
  80	181
  81	182	\subsection{Monadic Code}

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