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Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 24 |
1 files changed, 17 insertions, 7 deletions
@@ -5912,19 +5912,29 @@ The dotted notation can be used to write a function that accepts a variable number of arguments. To write a function that accepts variable arguments only, with no required arguments, use a single symbol. -The keyword symbol : can appear in the parameter list. It is not an argument, -but a separator between required parameters and optional parameters. -When the function is called, optional parameter for which arguments -are not supplied take on the value nil. +The keyword symbol : (colon) can appear in the parameter list. It is not an +argument, but a separator between required parameters and optional parameters. +When the function is called, optional parameter for which arguments are not +supplied take on the value nil. An optional parameter can also be written in the form (<name> <expr>). In this situation, if the call does not specify a value for the parameter, then the parameter takes on the value of the expression <expr>. -The expression is evaluated in the environment in which the lambda -was constructed. -Functions created by lambda capture the surrounding variable bindings. +The initializer expressions are evaluated an environment in which +all of the parameters are visible, which extends the environment in +which the lambda was constructed. For instance: + (let ((default 0)) + (lambda (str : (end (length str)) (counter default)) + (list str end counter))) + +In this lambda, the initializing expression for the optional parameter +end is (length str), and the str variable it refers to is the previous +argument. The initializer for the optional variable counter is +the expression default, and it refers to the binding established +by the surrounding let. This reference is captured as part of the +lambda's lexical closure. .TP Examples: |