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<TR><TD BGCOLOR="#2de52d"><DIV ALIGN=center><TABLE>
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<TR><TD><A NAME="htoc8"><B><FONT SIZE=6>Chapter 2</FONT></B></A></TD>
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<TD WIDTH="100%" ALIGN=center><B><FONT SIZE=6>Streams and parsers</FONT></B></TD>
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</TR></TABLE></DIV></TD>
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<A NAME="c:manstrm"></A>
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Objective Caml comprises a library type for streams (possibly infinite
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sequences of elements, that are evaluated on demand), and associated
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stream expressions, to build streams, and stream patterns, to
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destructure streams. Streams and stream patterns provide a natural
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approach to the writing of recursive-descent parsers.<BR>
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Streams are presented by the following extensions to the syntactic
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classes of expressions:
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| [< stream-component { ; stream-component } >]
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| parser [pattern] stream-matching
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| match expr with parser [pattern] stream-matching
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stream-matching ::= stream-pattern [pattern] -> expr
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{ | stream-pattern [pattern] -> expr }
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| [< stream-pat-comp
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{ ; stream-pat-comp [ ?? expr ] }>]
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' pattern [ when expr ]
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Stream expressions are bracketed by <CODE>[<</CODE> and <CODE>>]</CODE>. They
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represent the concatenation of their components. The component ' expr
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represents the one-element stream whose element is the value of
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expr. The component expr represents a sub-stream. For instance, if
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both s and t are streams of integers, then <CODE>[<'1; s; t; '2>]</CODE> is a
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stream of integers containing the element 1, then the elements of s,
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then those of t, and finally 2. The empty stream is denoted by <CODE>[< >]</CODE>.<BR>
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Unlike any other kind of expressions in the language, stream
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expressions are submitted to lazy evaluation: the components are not
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evaluated when the stream is built, but only when they are accessed
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during stream matching. The components are evaluated once, the first
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time they are accessed; the following accesses reuse the value
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computed the first time.<BR>
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Stream patterns, also bracketed by <CODE>[<</CODE> and <CODE>>]</CODE>, describe
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initial segments of streams. In particular, the stream pattern
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<CODE>[< >]</CODE> matches all streams. Stream pattern components are
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matched against the corresponding elements of a stream. The component
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<CODE>' pattern</CODE> matches the corresponding stream element against the
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pattern; if followed by when, the match is accepted only if the result
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of the guard expression is true. The component <CODE>pattern = expr</CODE>
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applies the function denoted by expr to the current stream, then
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matches the result of the function against pattern. Finally, the
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component ident simply binds the identifier to the stream being
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Stream matching proceeds destructively: once a component has been
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matched, it is discarded from the stream (by in-place modification).<BR>
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Stream matching proceeds in two steps: first, a pattern is selected by
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matching the stream against the first components of the stream
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patterns; then, the following components of the selected pattern are
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checked against the stream. If the following components do not match,
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the exception Stream.Error is raised. There is no backtracking here:
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stream matching commits to the pattern selected according to the first
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element. If none of the first components of the stream patterns match,
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the exception Stream.Failure is raised. The Stream.Failure exception
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causes the next alternative to be tried, if it occurs during the
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matching of the first element of a stream, before matching has
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committed to one pattern.<BR>
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The streams hold the count of their elements discarded. The optional
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pattern before the first stream pattern is bound to the stream count
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before the matching. The one after each stream pattern (optional, too)
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is bound to the stream count after the matching.<BR>
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The exception Stream.Error has a string parameter coming from the
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optional <CODE>?? expr</CODE> after the stream pattern components (its
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default is the empty string). This expression is evaluated only in
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<I><FONT COLOR=maroon>
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For remarks about Camlp4, write to:
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<img src="http://cristal.inria.fr/~ddr/images/email.jpg" alt=email align=top>
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