jaq is a clone of the JSON data processing tool [jq]. jaq aims to support a large subset of jq's syntax and operations.
jaq focusses on three goals:
Correctness:
jaq aims to provide a more correct and predictable implementation of jq,
while preserving compatibility with jq in most cases.
Examples of surprising jq behaviour
nan > nan is false, while nan < nan is true.[[]] | implode crashes jq, and this was not fixed at the time of writing despite
being known since five years.limit(n; exp) "extracts up to n outputs from exp".
This holds for values of n > 1, e.g. jq -n '[limit(2; 1, 2, 3)]' yields
[1, 2], but when n == 0, jq -n '[limit(0; 1, 2, 3)]' yields [1] instead of [].
And perhaps even worse, when n < 0, then limit yields all outputs from exp,
which is not documented.I drew inspiration from another Rust program, namely [jql]. However, unlike jql, jaq aims to closely imitate jq's syntax and semantics. This should allow users proficient in jq to easily use jaq.
To compile jaq, you need a Rust toolchain. See https://rustup.rs/ for instructions. (Note that Rust compilers shipped with Linux distributions may be too outdated to compile jaq.)
The following command installs jaq:
$ cargo install --locked jaq
$ cargo install --locked --git https://github.com/01mf02/jaq # latest development version
On my system, both commands place the executable at ~/.cargo/bin/jaq.
jaq should work on any system supported by Rust.
If it does not, please file an issue.
You may also install jaq using homebrew on macOS or Linux:
$ brew install jaq
$ brew install --HEAD jaq # latest development version
The following examples should give an impression of what jaq can currently do. You should obtain the same outputs by replacing jaq with jq. If not, your filing an issue would be appreciated. :) The syntax is documented in the [jq manual].
Access a field:
$ echo '{"a": 1, "b": 2}' | jaq '.a'
1
Add values:
$ echo '{"a": 1, "b": 2}' | jaq 'add'
3
Construct an array from an object in two ways and show that they are equal:
$ echo '{"a": 1, "b": 2}' | jaq '[.a, .b] == [.[]]'
true
Apply a filter to all elements of an array and filter the results:
$ echo '[0, 1, 2, 3]' | jaq 'map(.*2) | [.[] | select(. < 5)]'
[0, 2, 4]
Read (slurp) input values into an array and get the average of its elements:
$ echo '1 2 3 4' | jaq -s 'add / length'
2.5
Repeatedly apply a filter to itself and output the intermediate results:
$ echo '0' | jaq '[recurse(.+1; . < 3)]'
[0, 1, 2]
Lazily fold over inputs and output intermediate results:
$ seq 1000 | jaq -n 'foreach inputs as $x (0; . + $x)'
0 1 3 6 10 15 [...]
The following evaluation consists of several benchmarks that
allow comparing the performance of jaq, jq, and [gojq].
The empty benchmark runs n times the filter empty with null input,
serving to measure the startup time.
The bf-fib benchmark runs a Brainfuck interpreter written in jq,
interpreting a Brainfuck script that produces n Fibonacci numbers.
The other benchmarks evaluate various filters with n as input;
see bench.sh for details.
[jq-cff5336] was compiled manually with disabled assertion checking,
by adding -DNDEBUG to DEFS in Makefile.
I generated the benchmark data with bench.sh target/release/jaq jq-cff5336 gojq jq,
followed by pandoc -t gfm.
Table: Evaluation results in seconds ("N/A" if more than 10 seconds).
| Benchmark | n | jaq-0.9.0 | jq-cff5336 | gojq-0.12.9 | jq-1.6 | | ------------ | ------: | --------: | ---------: | ----------: | -----: | | empty | 512 | 0.83 | 1.25 | 0.96 | N/A | | bf-fib | 13 | 0.92 | 1.30 | 2.52 | 3.16 | | reverse | 1048576 | 0.08 | 1.09 | 1.16 | 1.54 | | sort | 1048576 | 0.20 | 1.53 | 1.77 | 1.81 | | add | 1048576 | 0.96 | 1.06 | 2.51 | 1.69 | | kv | 131072 | 0.33 | 0.25 | 0.49 | 0.49 | | kv-update | 131072 | 0.38 | 0.67 | N/A | N/A | | kv-entries | 131072 | 1.23 | 1.29 | 2.23 | 2.50 | | ex-implode | 1048576 | 1.32 | 1.59 | 1.73 | 2.77 | | reduce | 1048576 | 1.60 | 1.34 | N/A | 1.92 | | tree-flatten | 17 | 0.71 | 0.54 | 0.03 | 1.95 | | tree-update | 17 | 0.47 | 1.43 | 4.58 | 2.73 | | to-fromjson | 65536 | 0.09 | 1.59 | 0.16 | 1.69 |
The results show that jaq is
faster than jq-cff5336 on ten out of thirteen benchmarks and
faster than jq 1.6 on all benchmarks.
gojq is faster than jaq only on one benchmark, namely "tree-flatten"
(due to implementing the filter flatten natively instead of by definition).
Here is an overview that summarises:
Contributions to extend jaq are highly welcome.
.)..)if .a < .b then .a else .b end)reduce .[] as $x (0; . + $x), foreach .[] as $x (0; . + $x; . + .))try ... catch ...)@csv, @html, @json).[0], .a, .["a"]).[]).a?, .[]?).[3:7], .[0:-1])|). as $x | $x),)=)|=, +=, -=)//)or, and).a == .b, .a < .b)+, -, *, /, %)-)?)def map(f): [.[] | f];)def r: r; r)empty)error)inputs)length)floor, round, ceil)fromjson, tojson)explode, implode)ascii_downcase, ascii_upcase)split("foo"))reverse, sort, sort_by(-.))first, last, range, fold)range, recurse)sqrt, sin, log, pow, ...)startswith, ltrimstr, ...)group_by, min_by, max_by, ...)These filters are defined via more basic filters.
Their definitions are at std.jq.
null)true, false, not)nan, infinite, isnan, isinfinite, isfinite, isnormal)type)select(. >= 0))values, nulls, booleans, numbers, strings, arrays, objects, iterables, scalars)tostring, tonumber)map(.+1), map_values(.+1), add, join("a"), min, max)transpose, first, last, nth(10), flatten)to_entries, from_entries, with_entries)all, any)input)jaq currently does not aim to support several features of jq, such as:
jq uses 64-bit floating-point numbers (floats) for any number. By contrast, jaq interprets numbers such as 0 or -42 as machine-sized integers and numbers such as 0.0 or 3e8 as 64-bit floats. Many operations in jaq, such as array indexing, check whether the passed numbers are indeed integer. The motivation behind this is to avoid rounding errors that may silently lead to wrong results. For example:
$ jq -n '[0, 1, 2] | .[1.0000000000000001]'
1
$ jaq -n '[0, 1, 2] | .[1.0000000000000001]'
Error: cannot use 1.0 as integer
$ jaq -n '[0, 1, 2] | .[1]'
1
The rules of jaq are:
Examples:
$ jaq -n '1 + 2'
3
$ jaq -n '10 / 2'
5.0
$ jaq -n '1.0 + 2'
3.0
You can convert an integer to a floating-point number e.g.
by adding 0.0, by multiplying with 1.0, or by dividing with 1.
You can convert a floating-point number to an integer by
round, floor, or ceil:
$ jaq -n '1.2 | [floor, round, ceil]'
[1, 1, 2]
In jq, division by 0 has some surprising properties; for example,
0 / 0 yields nan, whereas
0 as $n | $n / 0 yields an error.
In jaq, n / 0 yields nan if n == 0, infinite if n > 0, and -infinite if n < 0.
jaq's behaviour is closer to the IEEE standard for floating-point arithmetic (IEEE 754).
jaq implements a total ordering on floating-point numbers to allow sorting values.
Therefore, it unfortunately has to enforce that nan == nan.
(jq gets around this by enforcing nan < nan, which breaks basic laws about total orders.)
Like jq, jaq prints nan and infinite as null in JSON,
because JSON does not support encoding these values as numbers.
jaq preserves fractional numbers coming from JSON data perfectly (as long as they are not used in some arithmetic operation), whereas jq may silently convert to 64-bit floating-point numbers:
$ echo '1e500' | jq '.'
1.7976931348623157e+308
$ echo '1e500' | jaq '.'
1e500
Therefore, unlike jq 1.6, jaq satisfies the following paragraph in the [jq manual]:
An important point about the identity filter is that it guarantees to preserve the literal decimal representation of values. This is particularly important when dealing with numbers which can't be losslessly converted to an IEEE754 double precision representation.
Please note that newer development versions of jq (e.g. commit cff5336) seem to preserve the literal decimal representation, even if it is not stated in the manual.
Like jq, jaq allows for assignments of the form p |= f.
However, jaq interprets these assignments differently.
Fortunately, in most cases, the result is the same.
In jq, an assignment p |= f first constructs paths to all values that match p.
Only then, it applies the filter f to these values.
In jaq, an assignment p |= f applies f immediately to any value matching p.
Unlike in jq, assignment does not explicitly construct paths.
jaq's implementation of assignment likely yields higher performance,
because it does not construct paths.
Furthermore, this also prevents several bugs in jq "by design".
For example, given the filter [0, 1, 2, 3] | .[] |= empty,
jq yields [1, 3], whereas
jaq yields [].
What happens here?
jq first constructs the paths corresponding to .[], which are .0, .1, .2, .3.
Then, it removes the element at each of these paths.
However, each of these removals changes the value that the remaining paths refer to.
That is, after removing .0 (value 0), .1 does not refer to value 1, but value 2!
That is also why value 1 (and in consequence also value 3) is not removed.
There is more weirdness ahead in jq;
for example, 0 | 0 |= .+1 yields 1 in jq,
although 0 is not a valid path expression.
However, 1 | 0 |= .+1 yields an error.
In jaq, any such assignment yields an error.
jaq attempts to use multiple outputs of the right-hand side, whereas
jq uses only the first.
For example, 0 | (., .) |= (., .+1) yields 0 1 1 2 in jaq,
whereas it yields only 0 in jq.
However, {a: 1} | .a |= (2, 3) yields {"a": 2} in both jaq and jq,
because an object can only associate a single value with any given key,
so we cannot use multiple outputs in a meaningful way here.
Because jaq does not construct paths,
it does not allow some filters on the left-hand side of assignments,
for example first, last, limit:
For example, [1, 2, 3] | first(.[]) |= .-1
yields [0, 2, 3] in jq, but is invalid in jaq.
Similarly, [1, 2, 3] | limit(2; .[]) |= .-1
yields [0, 1, 3] in jq, but is invalid in jaq.
(Inconsequentially, jq also does not allow for last.)
Like jq, jaq allows for the definition of filters, such as:
def map(f): [.[] | f];
However, unlike in jq, such filters in jaq cannot refer to themselves.
Furthermore, jaq does not support nested filters.
That is, a filter such as recurse cannot be defined in jaq:
def recurse(f): def r: ., (f | r); r;
Note that while recurse cannot be defined manually in jaq,
jaq provides recurse as core filter.
jq and jaq provide filters
reduce xs as $x (init; f) and
foreach xs as $x (init; f).
In jaq, the output of these filters is defined very simply:
Assuming that xs evaluates to x0, x1, ..., xn,
reduce xs as $x (init; f) evaluates to
~~~ init | x0 as $x | f | ... | xn as $x | f ~~~
and foreach xs as $x (init; f) evaluates to
~~~ text init | ., (x0 as $x | f | ... | ., (xn as $x | f )...) ~~~
This interpretation of reduce/foreach in jaq has the following advantages over jq:
f,
because it recurses only on the last of them,
although it outputs all of them.
foreach (5, 10) as $x (1; .+$x, -.) yields
6, -1, 9, 1 in jq, whereas it yields
1, 6, 16, -6, -1, 9, 1 in jaq.
Apart from the leading 1 (see below), we can see that
both jq and jaq yield the values 6 and -1
resulting from the first iteration (where $x is 5), namely
1 | 5 as $x | (.+$x, -.).
However, jq performs the second iteration (where $x is 10)
only on the last value returned from the first iteration, namely -1,
yielding the values 9 and 1 resulting from
-1 | 10 as $x | (.+$x, -.).
jaq yields these values too, but it also performs the second iteration
on all other values returned from the first iteration, namely 6,
yielding the values 16 and -6 that result from
6 | 10 as $x | (.+$x, -.).
reduce and foreach
special cases of the same code, reducing the potential for bugs.reduce and foreach.
In particular,
the values yielded by reduce ... are a subset of
the values yielded by foreach ... (where ... refers to xs as $x (init; f)).
Furthermore,
first(reduce ...) equals first(foreach ...), and
last(reduce ...) equals last(foreach ...).However, this interpretation comes at the cost of compatibility:
Most notably, the interpretation of foreach in jaq
differs from jq by yielding also the output of init.
For example, foreach (1, 2, 3) as $x (0; .+$x) yields
1, 3, 6 in jq and
0, 1, 3, 6 in jaq.
Furthermore, jq provides the filter
foreach xs as $x (init; f; proj) and interprets
foreach xs as $x (init; f) as
foreach xs as $x (init; f; .), whereas
jaq provides only
foreach xs as $x (init; f).
If you need the same behaviour in both jq and jaq,
including skipping the output of init,
you can replace foreach xs as $x (init; f; proj) by:
(foreach xs as $x ({y: init}; {x: $x, y: (.y | f)}) | select(has("x")) | .x as $x | .y | proj)
Note that it is much easier to simulate jq's behaviour in jaq than the other way. This suggests that jaq's behaviour is more general than that of jq.
-s / --slurp option),
jq combines the inputs of all files into one single array, whereas
jaq yields an array for every file.
The behaviour of jq can be approximated in jaq;
for example, to achieve the output of
jq -s . a b, you may use
jaq -s . <(cat a b).[(1,2) * (3,4)] yields [3, 6, 4, 8], whereas
[{a: (1,2), b: (3,4)} | .a * .b] yields [3, 4, 6, 8].
jaq yields [3, 4, 6, 8] in both cases.[0, 1] | .[3] = 3 yields [0, 1, null, 3]; that is,
jq fills up the list with nulls if we update beyond its size.
In contrast, jaq fails with an out-of-bounds error in such a case.input yields an error, whereas in jaq, it yields no output value.[x0, x1, ..., xn],
in jq, join(x) converts all elements of the input array to strings and intersperses them with x, whereas
in jaq, join(x) simply calculates x0 + x + x1 + x + ... + xn.
When all elements of the input array and x are strings, jq and jaq yield the same output.range(m; n) constructs a sequence of numbers m, m+1, ...,
where any number must be smaller than n.
In jq, m and n can be floating-point numbers, whereas
in jaq, m and n must be integers.
This is to avoid potential numerical stability problems.
That means that unlike in jq, you cannot use
range(m; infinite) to generate the infinite sequence m, m+1, ....
However, you can use m | recurse(.+1) to achieve the same in jaq.Contributions to jaq are welcome. In particular, implementing various filters of jq in jaq is a relatively low-hanging fruit.
To add a new core filter (such as group_by), it suffices to:
filter module.tests.rs,
and check whether jq yields the same results.Voilà!
Please make sure that after your change, cargo test runs successfully.
jaq has profited tremendously from: