(guile.info.gz) Source Properties

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 35.1 Source Properties
 ======================
 
 As Guile reads in Scheme code from file or from standard input, it
 remembers the file name, line number and column number where each
 expression begins.  These pieces of information are known as the
 "source properties" of the expression.  If an expression undergoes
 transformation -- for example, if there is a syntax transformer in
 effect, or the expression is a macro call -- the source properties are
 copied from the untransformed to the transformed expression so that, if
 an error occurs when evaluating the transformed expression, Guile's
 debugger can point back to the file and location where the expression
 originated.
 
    The way that source properties are stored means that Guile can only
 associate source properties with parenthesized expressions, and not, for
 example, with individual symbols, numbers or strings.  The difference
 can be seen by typing `(xxx)' and `xxx' at the Guile prompt (where the
 variable `xxx' has not been defined):
 
      guile> (xxx)
      standard input:2:1: In expression (xxx):
      standard input:2:1: Unbound variable: xxx
      ABORT: (unbound-variable)
      guile> xxx
      <unnamed port>: In expression xxx:
      <unnamed port>: Unbound variable: xxx
      ABORT: (unbound-variable)
 
 In the latter case, no source properties were stored, so the best that
 Guile could say regarding the location of the problem was "<unnamed
 port>".
 
    The recording of source properties is controlled by the read option
 named "positions" ( Reader options).  This option is switched
 _on_ by default, together with the debug options "debug" and
 "backtrace" ( Debugger options), when Guile is run
 interactively; all these options are _off_ by default when Guile runs a
 script non-interactively.
 

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