diff options
| -rw-r--r-- | txr.1 | 91 |
1 files changed, 84 insertions, 7 deletions
@@ -11318,6 +11318,83 @@ to the same object: the first occurrence creates the symbol and associates it with its name in a package. Subsequent occurrences do not create a new symbol, but retrieve the existing one. +.NP* Meta-Symbols, Meta-Numbers and Meta-Expressions + +The syntax of a symbol, integer or compound expression may be preceded by the +character +.codn @ . + +This is "meta syntax", whose meaning is unassigned as far as \*(TL +evaluation is concerned. It plays a syntactic role in the +.code op +operator, and in structural pattern matching. It also appears +in the quasiliteral notation. In other situations, application code may assign +meaning to meta syntax as the programmer sees fit. + +Meta syntax is defined as a shorthand notation, as follows: + +If +.code X +is a symbol or integer, the syntax +.code @X +is a shorthand for the compound expression +.codn "(sys:var X)" . +This is referred to as a +.I meta-symbol +if +.code X +is a symbol, or a +.I meta-number +if +.code X +is an integer. + +If +.code X +is a compound expression, either +.code "(...)" +or +.codn "[...]" , +then +.code @X +is a shorthand for +.codn "(sys:expr X)" . +This is called a +.IR meta-expression . + +The behavior of +.code @ +followed by the syntax of a floating-point constant introduced by a leading +decimal point, not preceded by digits, is unspecified. Examples of this +are +.code "@.123" +and +.codn "@.123E+5" . + +The behavior of +.code @ +followed by the syntax of a floating-point expression in exponential notation, +which lacks a decimal point, is also unspecified. An example of this is +.codn @12E5 . + +It is a syntax error for +.code @ +to be followed by what appears to be a floating-point constant consisting +of a decimal point flanked by digits on both sides. For instance +.code @1.2 +is rejected. + +A meta-expression followed by a period, and the syntax of another object is +otherwise interpreted as a referencing dot expression. For instance +.code @1.E3 +denotes +.code "(qref @1 E3)" +which, in turn, denotes +.codn "(qref (sys:var 1) E3)" , +even though the unprefixed character sequence +.code 1.E3 +is otherwise a floating-point constant. + .NP* Consing Dot Unlike other major Lisp dialects, \*(TL allows a consing dot with no forms @@ -12123,7 +12200,7 @@ may be understood according to the following transformations: [f a b c ... . x] --> [apply f a b c ... x] .brev -In addition to atoms, meta-expressions and meta-variables can appear in the dot +In addition to atoms, meta-expressions and meta-symbols can appear in the dot position, even though their underlying syntax is comprised of a compound expression. This appears to work according to a transformation pattern which superficially appears to be the same as that for atoms: @@ -40000,7 +40077,7 @@ for itself, and not the value of a variable. .mets >> @ symbol .syne .desc -A meta-variable can be used as a pattern expression. +A meta-symbol can be used as a pattern expression. This pattern unconditionally matches an object of any kind. The @@ -40818,7 +40895,7 @@ operator. Whenever the operator position of a pattern consists of a symbol which is not the name of a predicate operator, the expression denotes a predicate pattern, expected to conform to one of the first three syntax descriptions above. Whenever -the operator position of a pattern consists of a meta-variable, it is also a +the operator position of a pattern consists of a meta-symbol, it is also a predicate pattern, expected to conform to one of the second three syntax descriptions. @@ -40831,7 +40908,7 @@ may also appear in the dot position, rather than as an argument. The role of .meta avar and the consequences of omitting it are described below. -The second form of the predicate pattern consists of a meta-variable +The second form of the predicate pattern consists of a meta-symbol .meta rvar ("result variable") which must be a bindable symbol or else @@ -78033,8 +78110,8 @@ and .code do macros which was replaced starting in \*(TX 185. Also, this has the effect of disabling the special recognition of meta-expressions -and meta-variables in the dot position of function calls, and -the macro expansion of meta-variables in quasiliterals. This is +and meta-symbols in the dot position of function calls, and +the macro expansion of meta-symbols in quasiliterals. This is because the old .code op implementation implements these behaviors itself. The implication @@ -78485,7 +78562,7 @@ If the compatibility option is set to 124 or lower, the old behavior is restored. However, even without the presence of the compatibility option, if the .meta source -argument is a meta-expression or meta-variable (denotes by the +argument is a meta-expression or meta-symbol (denotes by the .code @ prefix in front of a compound expression or symbol, respectively) it is also treated in the old way. This latter behavior is obsolescent |