by chrisdone

chrisdone / jl

Functional sed for JSON

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jl ("JSON lambda") is a tiny functional language for querying and manipulating JSON.


$ jl 'map $ \o -> { sha: o.sha, ps: map _.sha o.parents }' x.json


Binary releases for Linux and OS X are available here.

Builds on Windows (see AppVeyor status), haven't added Windows binaries to the releases yet.

Installing from source:

  1. Get stack
  2. Run
    stack install
    in the repository directory.
  3. Add
    to your

Core syntax


123, 4.5, -6, "hi", null, true, false


\x -> y

Function application

get "f" o


x * (4 + 3)


{foo: 123, bar: 34.3, "a:b": "hi"}


[1, 4 * 5, id 5]


if x then y else z

Short-hand for fields:

o.f  is sugar for         get "f" o
_.f  is sugar for  (\o -> get "f" o)

For arrays:

_[0] is sugar for   (\o -> get 0 o)

Or objects:

_[k]     is sugar for   (\o -> get k o)
_["foo"] is sugar for   (\o -> get "foo" o)

Function composition:

a | b | c is sugar for `\x -> c (b (a x))`

Mini tutorial

You do everything with usual functional programming functions.

Returning the same thing, aka identity. That's normal in functional programming:

jl 'id'

A sequence of JSON strings will be read in and processed individually:


$ cat x.json | jl id

If you want to read the input in as an array, use

$ cat x.json | jl --array 'map _.a'

After processing, sometimes you want to print each element of the array out line by line, for that use

$ cat x.json | jl --array --lines 'map _.a'

Taking the first element of something, using syntax that looks like regular array access. The

is a short-hand so that you don't need a lambda:
jl '_[0]'

If you want to get what keys are available, you can run:

jl 'map keys | _[0]'

Taking the first element and then creating a record of some parts of it:

jl '_[0] | \o -> {msg: o.commit.message, n: o.commit.committer.name}'

Note the use of

to compose functions. Just like in the shell.

Applying a function to all elements in an array:

jl 'map _.commit.committer.name'

Note how you can nest property access easily.

Applying something more detailed, by constructing a record of our own

jl 'map $ \o -> {msg: o.commit.message, n: o.commit.committer.name}'

You can use

to avoid using parentheses on the right. That's a trick from Haskell.

Applying functions to nested data structures:

jl '_[0] | \o -> {msg: o.commit.message, n: o.commit.committer.name, ps: map _.html_url o.parents }'

Notice the

property comes by taking the
of all the parents.

Filtering is easy, simply write a function that returns true:

jl 'map (\o -> { sha: o.sha, ps: map _.sha o.parents }) | filter (\o -> length o.ps > 1)'

If you want to make an object with arbitrary keys that come at runtime, use

$ echo '"hello"' | jl '\x -> set x 123 {}'

This sets the key

in the empty object
with the value
. You can use
repeatedly to construct more keys.

If you want to construct an object from a list of key/values, you can use

$ echo '[{"k":"foo","v":123},{"k":"bar","v":456}]' | jl 'fold (\acc o -> set o.k o.v acc) {}'

Available functions

Record access

get :: JSON → JSON → JSON

Get the value at k from the object

set :: JSON → JSON → JSON → JSON

Set the value k to v in object

modify :: JSON → (JSON → JSON) → JSON → JSON

Modify the object at k with function f

keys :: JSON → JSON

Get all keys of the object

elems :: JSON → JSON

Get all elements of the object


map :: (JSON → JSON) → JSON → JSON

Apply a function to every element in the sequence

filter :: (JSON → JSON) → JSON → JSON

Keep only items from the sequence for which p returns true

takeWhile :: (JSON → JSON) → JSON → JSON

Take elements from a sequence while given predicate is true

empty :: JSON → JSON

Is a sequence empty?

length :: JSON → JSON

Get the length of a sequence

reverse :: JSON → JSON

Reverse a sequence

drop :: JSON → JSON → JSON

Drop n items from the sequence

elem :: JSON → JSON → JSON

Is x an element of y?

concat :: JSON → JSON

Concatenate a list of sequences into one sequence

zipWith :: (JSON → JSON → JSON) → JSON → JSON → JSON

Zip two lists calling with each element to f x y

take :: JSON → JSON → JSON

Take n items from sequence

fold :: (JSON → JSON → JSON) → JSON → JSON → JSON

Fold over a structure with a state.

dropWhile :: (JSON → JSON) → JSON → JSON

Drop elements from a sequence while a predicate is true

any :: (JSON → JSON) → JSON → JSON

Does p return true for any of the elements?

all :: (JSON → JSON) → JSON → JSON

Does p return true for all of the elements?

nub :: JSON → JSON

Return the sequence with no duplicates; the nub of it

sort :: JSON → JSON

Return the sequence sorted

append :: JSON → JSON → JSON

Append the members of the second sequence to the first sequence

sum :: JSON → JSON

Get the sum of a sequence

product :: JSON → JSON

Get the product of a sequence

minimum :: JSON → JSON

Get the minimum of a sequence

maximum :: JSON → JSON

Get the maximum of a sequence


words :: JSON → JSON

Split the string into a list of words

unwords :: JSON → JSON

Join the list of strings into a string separated by spaces

lines :: JSON → JSON

Split the string into a list of lines

unlines :: JSON → JSON

Join the list of strings into a string separated by lines and terminated by a new line

Predicate operators

/= :: JSON → JSON → JSON

a /= b


a = b

Boolean operators

&& :: JSON → JSON → JSON

a && b

|| :: JSON → JSON → JSON

a || b

not :: JSON → JSON

not b

Numeric operators


a > b


a < b

>= :: JSON → JSON → JSON

a >= b

<= :: JSON → JSON → JSON

a <= b


a * b


a + b


a - b


a / b

min :: JSON → JSON → JSON

a min b

max :: JSON → JSON → JSON

a max b

abs :: JSON → JSON

abs b

Function combinators

id :: JSON → JSON

Identity function, returns its input unchanged

compose :: (JSON → JSON) → (JSON → JSON) → JSON → JSON

Compose two functions

flip :: (JSON → JSON → JSON) → JSON → JSON → JSON

Flips the argument order of a function of two or more arguments

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