jaq

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.
  • The jq manual claims that 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.

• Performance: I created jaq originally because I was bothered by jq's long start-up time, which amounts to about 50ms on my machine. This can particularly show when processing of a large number of small files. jaq starts up about 30 times faster than jq and outperforms jq also on many other benchmarks.

• Simplicity: jaq aims to have a simple and small implementation, in order to reduce the potential for bugs and to facilitate contributions.

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.

Installation

From Source

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 (Note that Rust compilers shipped with Linux distributions may be too outdated to compile jaq.)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 jaq
$ cargo install --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.

Binaries

You may also install jaq using homebrew on macOS or Linux:

$ brew install jaq
$ brew install --HEAD jaq # latest development version

Examples

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]

Performance

The following benchmark compares the performance of jaq and jq 1.6. Each command is run via jq -n '<CMD>' and jaq -n '<CMD>', respectively.

I generated the benchmark data with bench.sh, followed by pandoc -t gfm.

Features

Here is an overview of the features already implemented and not yet implemented. Contributions to extend jaq are highly welcome.

Basics

• [x] Identity (.)
• [x] Recursion (..)
• [x] Basic data types (null, boolean, number, string, array, object)
• [x] if-then-else (if .a < .b then .a else .b end)
• [x] Folding (reduce .[] as $x (0, . + $x), foreach .[] as $x (0, . + $x))
• [ ] Error handling (try ... catch ...)
• [ ] String interpolation
• [ ] Format strings (@csv, @html, @json)

Paths

• [x] Indexing of arrays/objects (.[0], .a, .["a"])
• [x] Iterating over arrays/objects (.[])
• [x] Optional indexing/iteration (.a?, .[]?)
• [x] Array slices (.[3:7], .[0:-1])
• [x] String slices

Operators

• [x] Composition (|)
• [x] Binding (. as $x | $x)
• [x] Concatenation (,)
• [x] Plain assignment (=)
• [x] Update assignment (|=, +=, -=)
• [x] Alternation (//)
• [x] Logic (or, and)
• [x] Equality and comparison (.a == .b, .a < .b)
• [x] Arithmetic (+, -, *, /, %)
• [x] Negation (-)
• [x] Error suppression (?)

Definitions

• [x] Basic definitions (def map(f): [.[] | f];)
• [ ] Recursive definitions (def r: r; r)

Core filters

• [x] Empty (empty)
• [x] Errors (error)
• [x] Input (inputs)
• [x] Length (length)
• [x] Rounding (floor, round, ceil)
• [x] String <-> JSON (fromjson, tojson)
• [x] String <-> integers (explode, implode)
• [x] String normalisation (ascii_downcase, ascii_upcase)
• [x] String splitting (split("foo"))
• [x] Array filters (reverse, sort, sort_by(-.))
• [x] Stream consumers (first, last, range, fold)
• [x] Stream generators (range, recurse)
• [ ] More numeric filters (sqrt, sin, log, pow, ...)
• [ ] More string filters (startswith, ltrimstr, ...)
• [ ] More array filters (group_by, min_by, max_by, ...)

Standard filters

These filters are defined via more basic filters. Their definitions are at std.jq.

• [x] Undefined (null)
• [x] Booleans (true, false, not)
• [x] Special numbers (nan, infinite, isnan, isinfinite, isfinite, isnormal)
• [x] Type (type)
• [x] Filtering (select(. >= 0))
• [x] Selection (values, nulls, booleans, numbers, strings, arrays, objects, iterables, scalars)
• [x] Conversion (tostring, tonumber)
• [x] Iterable filters (map(.+1), map_values(.+1), add, join("a"), min, max)
• [x] Array filters (transpose, first, last, nth(10), flatten)
• [x] Object-array conversion (to_entries, from_entries, with_entries)
• [x] Universal/existential (all, any)
• [x] I/O (input)

Advanced features

jaq currently does not aim to support the advanced features of jq, such as:

• Modules
• Dates
• Regular expressions
• SQL-style operators
• Streaming

Differences between jq and jaq

Numbers

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:

• The sum, difference, product, and remainder of two integers is integer.
• Any other operation between two numbers yields a float.

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]

NaN and infinity

In jq, n / 0 yields nan (not a number) if n == 0 and fails otherwise. 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.

Preservation of fractional 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, 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.

Assignments

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.)

Definitions

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.

Miscellaneous

• Slurping: When files are slurped in (via the -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).
• Cartesian products: In jq, [(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.
• List updating: In jq, [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 reading: When there is no more input value left, in jq, input yields an error, whereas in jaq, it yields no output value.

Contributing

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:

1. Implement the filter in the filter module.
2. Add a test with the filter name to tests.rs, and check whether jq yields the same results.
3. Add derived filters to the standard library.

Voilà!

Please make sure that after your change, cargo test runs successfully.

Acknowledgements

jaq has profited tremendously from:

• serde_json to read and colored_json to output JSON,
• chumsky to parse and ariadne to pretty-print parse errors,
• mimalloc to boost the performance of memory allocation, and
• the Rust standard library, in particular its awesome Iterator, which builds the rock-solid base of jaq's filter execution