[pure-lang-svn] SF.net SVN: pure-lang:[721] pure/trunk/pure.1.in

[<ag...@us...>]

Revision: 721
          http://pure-lang.svn.sourceforge.net/pure-lang/?rev=721&view=rev
Author:   agraef
Date:     2008-09-05 22:57:51 +0000 (Fri, 05 Sep 2008)

Log Message:
-----------
Update documentation.

Modified Paths:
--------------
    pure/trunk/pure.1.in

Modified: pure/trunk/pure.1.in
===================================================================
--- pure/trunk/pure.1.in	2008-09-05 14:12:15 UTC (rev 720)
+++ pure/trunk/pure.1.in	2008-09-05 22:57:51 UTC (rev 721)
@@ -1813,6 +1813,86 @@
 [1,2,6,24,120,720,5040,40320,362880,3628800]
 .fi
 .PP
+.B Print syntax.
+As indicated, in interactive mode the normal forms of toplevel expressions are
+printed after each expression is entered. We also call this the
+.I read-eval-print
+loop. Normal form expressions are usually printed in the same form as you'd
+enter them. However, there are a few special kinds of objects like closures
+(anonymous and local functions), thunks (``lazy'' values to be evaluated when
+needed) and pointers which don't have a textual representation in the Pure
+syntax and will be printed in the format
+.BR "#<\fIobject description\fP>" .
+.PP
+The interpreter provides a ``hook'' to override the print representations of
+expressions at runtime by means of the
+.B __show__
+function. This is just an ordinary Pure function expected to return a string
+with the desired custom representation of a normal form value given as the
+function's single argument. __show__ is not defined by default, so you are
+free to add any rules that you want. The interpreter prints the strings
+returned by __show__ just as they are. It will
+.I not
+check whether they conform to Pure syntax and/or semantics, or modify them in
+any way.
+.PP
+Custom print representations are most useful for interactive purposes, when
+you're not happy with the default print syntax of some kinds of objects, or if
+you just want to change the format of numeric values. Here are some examples:
+.sp
+.nf
+> \fBusing\fP system;
+> __show__ x::double = sprintf "%0.6f" x;
+> 1/7;
+0.142857
+> __show__ x::int = sprintf "0x%0x" x;
+> 1786;
+0x6fa
+> \fBusing\fP math;
+> __show__ (x::double+:y::double) = sprintf "%0.6f+%0.6fi" (x,y);
+> cis (-pi);
+-1.000000+0.000000i
+.fi
+.PP
+The prelude function
+.BR str ,
+which returns the print representation of any Pure expression, uses __show__
+as well:
+.sp
+.nf
+> str (1/7);
+"0.142857"
+.fi
+.PP
+However, the str function always returns just the default representation of an
+expression if it is invoked through __show__. This prevents __show__ from
+going recursive, and allows you to define your custom representation in terms
+of the default one. E.g., the following rule removes the `L' suffixes from
+bigint values:
+.sp
+.nf
+> __show__ x::bigint = init (str x);
+> fact n = foldl (*) 1L (1..n);
+> fact 30;
+265252859812191058636308480000000
+.fi
+.PP
+By just purging the definition of the __show__ function you can easily go back
+to the standard print syntax:
+.sp
+.nf
+> \fBclear\fP __show__
+> 1/7; 1786; cis (-pi);
+0.142857142857143
+1786
+-1.0+:1.22460635382238e-16
+> str (1/7);
+"0.142857142857143"
+> fact 30;
+265252859812191058636308480000000L
+.fi
+.PP
+.B Interactive commands.
 When running interactively, the interpreter also accepts a number of special
 commands useful for interactive purposes. Here is a quick rundown of the
 currently supported operations:
@@ -1899,9 +1979,9 @@
 Note that these special commands are only recognized at the beginning of the
 interactive command line (they are not reserved keywords of the Pure
 language). Thus it's possible to ``escape'' identifiers looking like commands
-by entering a space at the start of the line. However, the compiler also warns
-you about identifiers which might be mistaken as command names, so that you
-can avoid this kind of problem.
+by entering a space at the beginning of the line. However, the compiler also
+warns you about identifiers which might be mistaken as command names, so that
+you can avoid this kind of problem.
 .PP
 Some commands which are especially important for effective operation of the
 interpreter are discussed in more detail in the following sections.
@@ -2273,11 +2353,11 @@
 and
 .B when
 clauses are tacked on to the end of the expression they belong to, which
-mimics mathematical notation but may be unfamilar if you're more accustomed to
-languages from the Algol/Pascal/C family. If you want to figure out what is
-actually going on there, it's usually best to read nested scopes ``in
-reverse'' (proceeding from the rightmost/outermost to the leftmost/innermost
-clause).
+mimics mathematical language but may be unfamilar if you're more accustomed to
+programming languages from the Algol/Pascal/C family. If you want to figure
+out what is actually going on there, it's usually best to read nested scopes
+``in reverse'' (proceeding from the rightmost/outermost to the
+leftmost/innermost clause).
 .PP
 Also note that since
 .B with


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