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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 26 |
1 files changed, 25 insertions, 1 deletions
@@ -508,6 +508,19 @@ If the variable is followed by a regular expression directive, the extent is determined by finding the closest match for the regular expression. (See Regular Expressions section below). +.SS Special Symbols + +Just like in the programming language Lisp, the names nil and t cannot be used +as variables. They always represent themselves, and have many uses, internal to +the program as well as externally visible. The nil symbol stands for the empty +list object, an object which marks the end of a list, and boolean false. It is +synonymous with the syntax () which may be used interchangeably with nil in +most constructs. + +Names whose names begin with the : character are keyword symbols. These also +may not be used as variables either and stand for themselves. Keywords are +useful for labeling information and situations. + .SS Consecutive Variables If an unbound variable is followed by another unbound variable, the @@ -1518,7 +1531,9 @@ instance will bind the string "ab\tc" (the letter a, b, a tab character, and c) to the variable A if A is unbound. If A is bound, this will fail unless -A already contains an identical string. +A already contains an identical string. However, the right hand side of +cannot be an unbound variable, nor a complex expression that contains unbound +variables. The left hand side of a bind can be a nested list pattern containing variables. The last item of a list at any nesting level can be preceded by a dot, which @@ -1536,6 +1551,15 @@ binds H to "how", N to "now", B to "brown" and C to "cow". The dot notation may be used at any nesting level. it must be preceded and followed by a symbol: the forms (.) (. X) and (X .) are invalid. +The number of items in a left pattern match must match the number of items in +the corresponding right side object. So the pattern () only matches +an empty list. The notation () and nil means exactly the same thing. + +The symbols nil, t and keyword symbols may be used on either side. +They represent themselves. For example @(bind :foo :bar) fails, +but @(bind :foo :foo) succeeds since the two sides denote the same +keyword symbol object. + .SH BLOCKS .SS Introduction |