diff options
| -rw-r--r-- | txr.1 | 54 |
1 files changed, 31 insertions, 23 deletions
@@ -11764,36 +11764,46 @@ 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 +.NP* Meta-Atoms and Meta-Expressions -The syntax of a symbol, integer or compound expression may be preceded by the -character -.codn @ . +An expression may be preceded by the +.code @ +(at sign) character. If the expression is an +.codn atom , +then this is a meta-atom, otherwise it is a meta-expression. + +When the atom is a symbol, this is also called a meta-symbol and in situations +when such a symbol behaves like a variable, it is also referred to as a +meta-variable. -This is "meta syntax", whose meaning is unassigned as far as \*(TL -evaluation is concerned. It plays a syntactic role in the +When the atom is an integer, the meta-atom expression is called a meta-number. + +Meta-atom and meta-expression expressions have no evaluation semantics; +evaluating them throws an exception. They 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. +operator, which makes use of meta-variables and meta-numbers, and in structural +pattern matching, which uses meta-variables as pattern variables and whose +operator vocabulary is based on meta-expressions. + +Meta-expressions also appear 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 +is the syntax of an atom, such as a symbol, string or vector, then .code @X -is a shorthand for the compound expression +is a shorthand for the 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. +Here, +.code sys:var +refers to the +.code var +symbol in the +.codn system-package . If .code X @@ -11803,10 +11813,8 @@ or .codn "[...]" , then .code @X -is a shorthand for +is a shorthand for the expression .codn "(sys:expr X)" . -This is called a -.IR meta-expression . The behavior of .code @ |