Introspection: Difference between revisions
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=={{header|jq}}== |
=={{header|jq}}== |
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jq's powers of introspection are currently very limited, being |
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essentially confined to the built-in function `builtins`, which |
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as the name suggests only yields information about built-in filters (name and arity). |
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'''Works with jq, the C implementation of jq''' |
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Version information can however be made available to a running jq program as illustrated here: |
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'''Works with gojq, the Go implementation of jq''' |
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| ⚫ | |||
jq's support for introspection is limited. For example, jq's version |
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References to undefined functions and undefined variables (that is jq's "$-variables") |
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number is available from the command-line but not as a built-in; and |
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are regarded as errors that cannot be caught, but in a pinch one can use the |
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it is not in general possible to check whether a variable has been |
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technique illustrated here: |
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defined without causing an error. However, jq does make it possible |
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for a program to check whether a variable has been defined globally, |
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so we'll go with that. |
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Note that jq's `builtins` function provides details about jq's built-in |
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jq -n --argjson bloop null 'if $bloop then $bloop|length else "undefined" end' |
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functions, and similarly the `modulemeta` function provides some support for |
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introspection within modules, but otherwise there is no support for |
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determining the functions that have been user-defined. |
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Assuming JQ has been defined appropriately as a shell. variable, one could |
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As it happens, jq's "abs" function is named "length" (don't ask why), |
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invoke jq with a global "bloop" variable as follows: |
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so the task regarding `abs()` cannot really be accomplished using the name `abs`. |
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<pre> |
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$JQ -n --arg version "$($JQ --version)" --argjson bloop -3 -f introspection.jq |
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</pre> |
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The following program has been tested with recent versions of both jq |
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and gojq, but may fail with earlier versions. Note also that |
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jq's `length` has the semantics of `abs` when applied to numbers, |
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so if your jq does not support `abs` by that name, you could use `length`. |
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<syntaxhighlight lang="jq"> |
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def checkVersion: |
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$version |
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| capture("^[^- ]*(?<jq>[- ])(?<major>[0-9]*)[.](?<minor>[0-9]*)") // {jq: 0} |
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| if .jq == 0 then "unrecognized version identification: \($version)" | error |
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elif .jq == "-" # jq |
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and (.major < "1" or (.major == "1" and .minor < "5")) |
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then "version \($version) is too old" | error |
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elif .jq == " " # gojq |
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and (.major == "0" and (.minor | tonumber) < 12) |
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then "version \($version) is too old" | error |
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else . |
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end; |
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checkVersion |
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# Check that abs/0 is defined |
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| (builtins | index("abs/0")) as $ix |
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| if $ix == null then "abs/0 not available" | error else . end |
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# Is bloop a global variable? |
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| if ($ARGS.named | has("bloop")) then $ARGS.named["bloop"] | abs |
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else "There is no globally defined variable name $bloop." | error |
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end |
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</syntaxhighlight> |
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Examples: |
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{{output}} |
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<pre> |
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$ gojq -n --arg version "$(gojq --version)" --argjson bloop -3 -f introspection.jq |
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3 |
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| ⚫ | |||
jq: error (at <unknown>): There is no globally defined variable name bloop. |
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</pre> |
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=={{header|Jsish}}== |
=={{header|Jsish}}== |
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