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Improve manual in various ways (inputs, sort_by, foreach sections, et…
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…c.) (#2744)

- Add error/0 and mentions null input behavior (close #2231)
- Explain value iterator suffix syntax .foo[] (close #1047)
- Mention array slicing is also zero-based (close #2094)
- Add examples of input and inputs filters (close #2216, close #2470)
- Improve sort_by about multiple values (close #2103, close #2467, close #2474)
- Improve foreach section and simplify examples (close #1148, close #2169)
- Fix recurse/1 document on how it is identical using recurse/2 (close #2036, close #2412)
- Add non-string examples of index/1, rindex/1 (close #1422)
- Simplify the example of truncate_stream/1 (close #1736)
@itchyny
itchyny authored Jul 24, 2023
1 parent 1cf6515 commit ed334b5
Showing 9 changed files with 698 additions and 302 deletions.
2 changes: 1 addition & 1 deletion docs/content/download/default.yml
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@@ -155,7 +155,7 @@ body:
You can build it using the usual `./configure && make && sudo
make install` rigmarole.
If you're interested in using the lastest development version, try:
If you're interested in using the latest development version, try:
git clone --recursive https://github.com/jqlang/jq.git
cd jq
206 changes: 140 additions & 66 deletions docs/content/manual/manual.yml
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Original file line number Diff line number Diff line change
@@ -79,8 +79,8 @@ sections:
jq filters run on a stream of JSON data. The input to jq is
parsed as a sequence of whitespace-separated JSON values which
are passed through the provided filter one at a time. The
output(s) of the filter are written to standard out, again as a
sequence of whitespace-separated JSON data.
output(s) of the filter are written to standard output, as a
sequence of newline-separated JSON data.
Note: it is important to mind the shell's quoting rules. As a
general rule it's best to always quote (with single-quote
@@ -132,7 +132,7 @@ sections:
* `--stream-errors`:
Like `--stream`, but invalid JSON inputs yield array calues
Like `--stream`, but invalid JSON inputs yield array values
where the first element is the error and the second is a path.
For example, `["a",n]` produces ["Invalid literal at line 1,
column 9",[1]]`.
@@ -334,7 +334,7 @@ sections:
input, jq processing can sometimes make it appear as though
it does. For example, using the current implementation of
jq, we would see that the expression:
1E1234567890 | .
produces `1.7976931348623157e+308` on at least one platform.
@@ -372,7 +372,7 @@ sections:
output: ['"Hello, world!"']

- program: '.'
input: '0.12345678901234567890123456789'
input: '0.12345678901234567890123456789'
output: ['0.12345678901234567890123456789']

- program: '[., tojson]'
@@ -398,7 +398,7 @@ sections:
JSON object (aka dictionary or hash) as input, `.foo` produces
the value at the key "foo" if the key is present, or null otherwise.
A filter of the form `.foo.bar` is equivalent to `.foo|.bar`.
A filter of the form `.foo.bar` is equivalent to `.foo | .bar`.
The `.foo` syntax only works for simple, identifier-like keys, that
is, keys that are all made of alphanumeric characters and
@@ -484,6 +484,7 @@ sections:
Either index may be negative (in which case it counts
backwards from the end of the array), or omitted (in which
case it refers to the start or end of the array).
Indices are zero-based.
examples:
- program: '.[2:4]'
@@ -509,7 +510,8 @@ sections:
entirely, it will return *all* of the elements of an
array. Running `.[]` with the input `[1,2,3]` will produce the
numbers as three separate results, rather than as a single
array.
array. A filter of the form `.foo[]` is equivalent to
`.foo | .[]`.
You can also use this on an object, and it will return all
the values of the object.
@@ -525,6 +527,10 @@ sections:
input: '[]'
output: []

- program: '.foo[]'
input: '{"foo":[1,2,3]}'
output: ['1','2','3']

- program: '.[]'
input: '{"a": 1, "b": 1}'
output: ['1', '1']
@@ -533,7 +539,8 @@ sections:
body: |
Like `.[]`, but no errors will be output if . is not an array
or object.
or object. A filter of the form `.foo[]?` is equivalent to
`.foo | .[]?`.
- title: "Comma: `,`"
body: |
@@ -728,15 +735,15 @@ sections:
Recursively descends `.`, producing every value. This is the
same as the zero-argument `recurse` builtin (see below). This
is intended to resemble the XPath `//` operator. Note that
`..a` does not work; use `..|.a` instead. In the example
below we use `..|.a?` to find all the values of object keys
`..a` does not work; use `.. | .a` instead. In the example
below we use `.. | .a?` to find all the values of object keys
"a" in any object found "below" `.`.
This is particularly useful in conjunction with `path(EXP)`
(also see below) and the `?` operator.
examples:
- program: '..|.a?'
- program: '.. | .a?'
input: '[[{"a":1}]]'
output: ['1']

@@ -1190,12 +1197,25 @@ sections:
input: 'null'
output: ['[1,2,3]']

- title: "`error(message)`"
- title: "`error`, `error(message)`"
body: |
Produces an error, just like `.a` applied to values other than
null and objects would, but with the given message as the
error's value. Errors can be caught with try/catch; see below.
Produces an error with the input value, or with the message
given as the argument. Errors can be caught with try/catch;
see below.
When the error value is `null`, it produces nothing and works
just like `empty`. So `[null | error]` and `[error(null)]` both
emit `[]`.
examples:
- program: 'try error catch .'
input: '"error message"'
output: ['"error message"']

- program: 'try error("invalid value: \(.)") catch .'
input: '42'
output: ['"invalid value: 42"']

- title: "`halt`"
body: |
@@ -1481,19 +1501,25 @@ sections:
sorted order), and if their keys are equal then the values
are compared key by key.
`sort` may be used to sort by a particular field of an
object, or by applying any jq filter.
`sort_by(f)` compares two elements by comparing the result of
`f` on each element.
`sort_by` may be used to sort by a particular field of an
object, or by applying any jq filter. `sort_by(f)` compares
two elements by comparing the result of `f` on each element.
When `f` produces multiple values, it firstly compares the
first values, and the second values if the first values are
equal, and so on.
examples:
- program: 'sort'
input: '[8,3,null,6]'
output: ['[null,3,6,8]']

- program: 'sort_by(.foo)'
input: '[{"foo":4, "bar":10}, {"foo":3, "bar":100}, {"foo":2, "bar":1}]'
output: ['[{"foo":2, "bar":1}, {"foo":3, "bar":100}, {"foo":4, "bar":10}]']
input: '[{"foo":4, "bar":10}, {"foo":3, "bar":10}, {"foo":2, "bar":1}]'
output: ['[{"foo":2, "bar":1}, {"foo":3, "bar":10}, {"foo":4, "bar":10}]']

- program: 'sort_by(.foo, .bar)'
input: '[{"foo":4, "bar":10}, {"foo":3, "bar":20}, {"foo":2, "bar":1}, {"foo":3, "bar":10}]'
output: ['[{"foo":2, "bar":1}, {"foo":3, "bar":10}, {"foo":3, "bar":20}, {"foo":4, "bar":10}]']

- title: "`group_by(path_expression)`"
body: |
@@ -1620,9 +1646,21 @@ sections:
- program: 'index(", ")'
input: '"a,b, cd, efg, hijk"'
output: ['3']
- program: 'index(1)'
input: '[0,1,2,1,3,1,4]'
output: ['1']
- program: 'index([1,2])'
input: '[0,1,2,3,1,4,2,5,1,2,6,7]'
output: ['1']
- program: 'rindex(", ")'
input: '"a,b, cd, efg, hijk"'
output: ['12']
- program: 'rindex(1)'
input: '[0,1,2,1,3,1,4]'
output: ['5']
- program: 'rindex([1,2])'
input: '[0,1,2,3,1,4,2,5,1,2,6,7]'
output: ['8']

- title: "`inside`"
body: |
@@ -1846,7 +1884,7 @@ sections:
When called without an argument, `recurse` is equivalent to
`recurse(.[]?)`.
`recurse(f)` is identical to `recurse(f; . != null)` and can be
`recurse(f)` is identical to `recurse(f; true)` and can be
used without concerns about recursion depth.
`recurse(f; condition)` is a generator which begins by
@@ -2861,29 +2899,6 @@ sections:
$times_three | [. + $times_three]) | ...`: here the binding
`$times_three` is _not_ visible past the closing parenthesis.
- title: Reduce
body: |
The `reduce` syntax in jq allows you to combine all of the
results of an expression by accumulating them into a single
answer. As an example, we'll pass `[3,2,1]` to this expression:
reduce .[] as $item (0; . + $item)
For each result that `.[]` produces, `. + $item` is run to
accumulate a running total, starting from 0. In this
example, `.[]` produces the results 3, 2, and 1, so the
effect is similar to running something like this:
0 | (3 as $item | . + $item) |
(2 as $item | . + $item) |
(1 as $item | . + $item)
examples:
- program: 'reduce .[] as $item (0; . + $item)'
input: '[10,2,5,3]'
output: ['20']

- title: "`isempty(exp)`"
body: |
@@ -2894,6 +2909,14 @@ sections:
input: 'null'
output: ['true']

- program: 'isempty(.[])'
input: '[]'
output: ['true']

- program: 'isempty(.[])'
input: '[1,2,3]'
output: ['false']

- title: "`limit(n; exp)`"
body: |
@@ -2931,32 +2954,79 @@ sections:
input: '10'
output: ['[0,9,5]']

- title: "`reduce`"
body: |
The `reduce` syntax allows you to combine all of the results of
an expression by accumulating them into a single answer.
The form is `reduce EXP as $var (INIT; UPDATE)`.
As an example, we'll pass `[1,2,3]` to this expression:
reduce .[] as $item (0; . + $item)
For each result that `.[]` produces, `. + $item` is run to
accumulate a running total, starting from 0 as the input value.
In this example, `.[]` produces the results `1`, `2`, and `3`,
so the effect is similar to running something like this:
0 | 1 as $item | . + $item |
2 as $item | . + $item |
3 as $item | . + $item
examples:
- program: 'reduce .[] as $item (0; . + $item)'
input: '[1,2,3,4,5]'
output: ['15']

- program: 'reduce .[] as [$i,$j] (0; . + $i * $j)'
input: '[[1,2],[3,4],[5,6]]'
output: ['44']

- program: 'reduce .[] as {$x,$y} (null; .x += $x | .y += [$y])'
input: '[{"x":"a","y":1},{"x":"b","y":2},{"x":"c","y":3}]'
output: ['{"x":"abc","y":[1,2,3]}']

- title: "`foreach`"
body: |
The `foreach` syntax is similar to `reduce`, but intended to
allow the construction of `limit` and reducers that produce
intermediate results (see example).
intermediate results.
The form is `foreach EXP as $var (INIT; UPDATE; EXTRACT)`.
Like `reduce`, `INIT` is evaluated once to produce a state
value, then each output of `EXP` is bound to `$var`, `UPDATE`
is evaluated for each output of `EXP` with the current state
and with `$var` visible. Each value output by `UPDATE`
replaces the previous state. Finally, `EXTRACT` is evaluated
for each new state to extract an output of `foreach`.
As an example, we'll pass `[1,2,3]` to this expression:
foreach .[] as $item (0; . + $item; [$item, . * 2])
Like the `reduce` syntax, `. + $item` is run for each result
that `.[]` produces, but `[$item, . * 2]` is run for each
intermediate values. In this example, since the intermediate
values are `1`, `3`, and `6`, the `foreach` expression produces
`[1,2]`, `[2,6]`, and `[3,12]`. So the effect is similar
to running something like this:
This is mostly useful only for constructing `reduce`- and
`limit`-like functions. But it is much more general, as it
allows for partial reductions (see the example below).
0 | 1 as $item | . + $item | [$item, . * 2],
2 as $item | . + $item | [$item, . * 2],
3 as $item | . + $item | [$item, . * 2]
When `EXTRACT` is omitted, the identity filter is used.
That is, it outputs the intermediate values as they are.
examples:
- program: '[foreach .[] as $item
([[],[]];
if $item == null then [[],.[0]] else [(.[0] + [$item]),[]] end;
if $item == null then .[1] else empty end)]'
input: '[1,2,3,4,null,"a","b",null]'
output: ['[[1,2,3,4],["a","b"]]']
- program: 'foreach .[] as $item (0; . + $item)'
input: '[1,2,3,4,5]'
output: ['1','3','6','10','15']

- program: 'foreach .[] as $item (0; . + $item; [$item, . * 2])'
input: '[1,2,3,4,5]'
output: ['[1,2]','[2,6]','[3,12]','[4,20]','[5,30]']

- program: 'foreach .[] as $item (0; . + 1; {index: ., $item})'
input: '["foo", "bar", "baz"]'
output:
- '{"index":1,"item":"foo"}'
- '{"index":2,"item":"bar"}'
- '{"index":3,"item":"baz"}'

- title: Recursion
body: |
@@ -3098,19 +3168,23 @@ sections:
Outputs one new input.
Note that when using `input` it is generally be necessary to
invoke jq with the -n command-line option, otherwise
invoke jq with the `-n` command-line option, otherwise
the first entity will be lost.
echo 1 2 3 4 | jq '[., input]' # [1,2] [3,4]
- title: "`inputs`"
body: |
Outputs all remaining inputs, one by one.
This is primarily useful for reductions over a program's
inputs. Note that when using `inputs` it is generally necessary
to invoke jq with the -n command-line option, otherwise
to invoke jq with the `-n` command-line option, otherwise
the first entity will be lost.
echo 1 2 3 | jq -n 'reduce inputs as $i (0; . + $i)' # 6
- title: "`debug`, `debug(msgs)`"
body: |
@@ -3188,9 +3262,9 @@ sections:
given streaming expression.
examples:
- program: '[1|truncate_stream([[0],1],[[1,0],2],[[1,0]],[[1]])]'
- program: 'truncate_stream([[0],1],[[1,0],2],[[1,0]],[[1]])'
input: '1'
output: ['[[[0],2],[[0]]]']
output: ['[[0],2]', '[[0]]']

- title: "`fromstream(stream_expression)`"
body: |
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