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Overview
| Comment: | Tip 508: Clarify behavior of default value with read traces |
|---|---|
| Downloads: | Tarball | ZIP archive | SQL archive |
| Timelines: | family | ancestors | descendants | both | trunk |
| Files: | files | file ages | folders |
| SHA3-256: |
d894c3b20e7878e39cdf3934fe693a17 |
| User & Date: | fbonnet 2018-06-09 21:04:59 |
Context
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2018-06-10
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| 20:44 | Formatting tweaking check-in: 11ef41277a user: dkf tags: trunk | |
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2018-06-09
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| 21:04 | Tip 508: Clarify behavior of default value with read traces check-in: d894c3b20e user: fbonnet tags: trunk | |
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2018-06-06
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| 19:46 | Changed e-mail check-in: 9da553226b user: rene tags: trunk | |
Changes
Changes to tip/508.md.
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existing scalar variable or an invalid array name. * <code>array default exists <em>arrayName</em></code>: Test whether _arrayName_ has a default value. Return false if _arrayName_ does not exist. Raise an error if _arrayName_ is not an array variable. * <code>array default unset <em>arrayName</em></code>: Unset _arrayName_ default value. Does nothing if _arrayName_ does not exist. Raise an error if _arrayName_ is not an array variable. # Examples Initial state: % array exists var 0 ................................................................................ variable: % array exists var2 0 % array default set var2 1234 % array exists var2 1 # Implementation The proposed implementation is available on branch [tip-508](https://core.tcl.tk/tcl/timeline?r=tip-508) in the Tcl Fossil repository. # Copyright This document has been placed in the public domain. |
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existing scalar variable or an invalid array name. * <code>array default exists <em>arrayName</em></code>: Test whether _arrayName_ has a default value. Return false if _arrayName_ does not exist. Raise an error if _arrayName_ is not an array variable. * <code>array default unset <em>arrayName</em></code>: Unset _arrayName_ default value. Does nothing if _arrayName_ does not exist. Raise an error if _arrayName_ is not an array variable. The default value is only used as a last resort where an error would normally occur when accessing a non-existing array element. Read traces are processed before that point and are free to set the element value or modify the default value. # Examples Initial state: % array exists var 0 ................................................................................ variable: % array exists var2 0 % array default set var2 1234 % array exists var2 1 The default value can be set, modified or unset by trace handlers: % set var(foo) can't read "var(foo)": no such element in array % trace add variable var read setRandomDefault % proc setRandomDefault {name1 name2 op} { upvar $name1 arr array default set arr [expr {rand()}] } % set var(foo) 0.9132881643778124 % set var(foo) 0.6341786978925479 % set var(foo) 0.5976932461362766 % array default get var 0.5976932461362766 % proc setRandomDefault {name1 name2 op} { upvar $name1 arr array default unset arr } % set var(foo) can't read "var(foo)": no such element in array # Implementation The proposed implementation is available on branch [tip-508](https://core.tcl.tk/tcl/timeline?r=tip-508) in the Tcl Fossil repository. # Copyright This document has been placed in the public domain. |