The matcher arrow is often very useful when you are sure two things will match, but you want to use the matcher to examine the contents of one of them. That way you don't use a conditional expression of the form:
if ... matches ... then ....Instead we use the matcher arrow "-->". "-->" should not be confused with "->". The latter is the assignment arrow.
The infix operation "-->" could have been defined thus:
define 8 datum --> pattern;
unless datum matches pattern then
mishap('NON MATCHING ARGUMENTS FOR -->',[%datum,pattern%])
endunless
enddefine;
The `8' in the procedure heading indicates that an infix operation
of precedence 8 is being defined.
This definition looks as if it doesn't do anything when the datum matches the pattern. We shall see that, on the contrary, it can be used to `decompose' a list.
The simplest use of `-->' is to check that a list has a certain format:
list --> [junction == ];checks that LIST starts with "JUNCTION". If not, an error occurs.
[a b c d] --> [junction ==];
;;; MISHAP - NON MATCHING ARGUMENTS FOR -->
;;; INVOLVING: [a b c d] [junction ==]
;;; DOING : --> compile
The operation --> can also be used to decompose a list, i.e. to assign
some of its components to variables.
vars first, second, rest;
vars list = [dogs like wagging their tails];
list --> [?first ?second ??rest];
first =>
** dogs
second =>
** like
rest =>
** [wagging their tails]
That would have caused an error if LIST had had fewer than two elements.
Since there are at least two elements, it gives FIRST the first element,
as its value, SECOND the second element, and REST a list containing all
the remainder.
Here's another example.
vars first, second, rest;
[mary had a little lamb] --> [?first ?second ??rest];
first =>
** mary
second =>
** had
rest =>
** [a little lamb]
Notice the different effects of "?" and "??". The former means, roughly
"match ONE element", the latter means, roughly "match any number of
elements". The use of "??" and "==" can mean that there are alternative
ways of matching a list againt a pattern. In that case the matcher will
find only one of them. Which one it finds is not defined. The Birmingham
ftp directory has a library called "doesmatch" that finds all the
possible matches.