rq

rq is a tiny functional language with which you can manipulate JSON. Basically, it is (an insignificant subset of!) jq, written in Rust.

NOTE: This project is in its very early stages; lot's of essential functions—and perhaps even syntax—might be missing, and overall I can't guarantee that anything actually works. Use at your own risk :)

Installation

Use cargo install. For nix users, a dev-shell is provided by the flake; one can access it with nix develop. Additionally, you can run rq directly from the git repository:

$ nix run github:slotThe/rq

Usage

Call rq with an expression, and pipe some JSON into it!

$ cat test.json
[{"name": "John Doe", "age": 43, "phones": ["+44 1234567", "+44 2345678"]}]

$ cat test.json | rq '\x -> x.0.phones.1'
+44 2345678

Some more usage examples:

$ cat simple.json
[{"name": "John Doe", "age": 43, "phone": +44 1234567"},{"name":"Alice"},{"name":"Bob", "age":42}]

$ cat simple.json | rq 'map .name'
["John Doe","Alice","Bob"]

$ cat simple.json | rq 'map .age | foldl (+) 0'
85

$ cat simple.json | rq 'filter (get "age" | (>= 42)) | map (\x -> { x.name: x.age })'
[{"John Doe":43},{"Bob":42}]

$ cargo metadata --format-version=1 | rq '.packages | map .name'
["ahash", "aho-corasick", "allocator-api2", "anyhow", "ariadne", "cc", "cfg-if", "chumsky", "hashbrown", "libc", "memchr", "once_cell", "proc-macro2", "psm", "quote", "regex-automata", "regex-syntax", "rq", "serde", "serde_derive", "stacker", "syn", "unicode-ident", "unicode-width", "version_check", "winapi", "winapi-i686-pc-windows-gnu", "winapi-x86_64-pc-windows-gnu", "yansi", "zerocopy", "zerocopy-derive"]

The expression language

• Constants: null, false, true, 1, 2.6, "string".

• Lambdas, which can be written in various ways:

    \x -> x        |x| x        λx → x
  

• Application is done via whitespace: (\x -> x 1 2) const. This would be akin to

    (|x| x(1, 2))(const)
  

  in pseudo-Rust notation (const is a builtin function).

• Binary operations:

  • Arithmetic operations, with the usual precedence rules of * and / being preferred over + and -:

      1 + 3 * 5 + 4 - 7    ≡    ((1 + (3 * 5)) + 4) - 7
    

    Additionally + also concatenates strings.

      "furble" + "wurble"    ≡    "furblewurble"
    

  • Comparison operations:

      1 = 3 * 5 + 4 - 7    ≡    1 = (((3 * 5) + 4) - 7)
    
      1 < 4 + 5 = 5        ≡    (1 < (4 + 5)) = 5
    

  The following table details the precedence rules:

  Op	Precedence
  *, /	3
  +, -	2
  =, !=, <, <=, >, >=	1

• If-then-else expressions:

    if 5 = 2 + 3 then "wurble" else 4  ≡  if (5 = (2 + 3)) then "wurble" else 4
                                       ≡  "wurble"
  

• Arrays: [1, 3, null]. Arrays can contain arbitrary expressions:

    λx → [1, get 0 x, if false then 1 else 5]
  

• Objects: { "this": 3, "that": null }. Apostrophes can be omitted:

    { this: 3, that: null }    ≡    { "this": 3, "that": null }
  

  In fact, keys can be arbitrary expressions—just make sure they actually evaluate to something sensible!

    { if true then "this" else "thus": 3, that: null }
      ≡  { "this": 3, "that": null }
  

• Expressions can be type-annotated with :: or ∷, though this is usually not necessary.

    λ> 1 + 2 :: JSON
    3
  
    λ> :e 1 + 2 + (3 ∷ JSON)
    + (+ 1 2) (3 ∷ JSON)
  

Syntactic sugar

• The get function—with which one can index arrays and objects—can be abbreviated by .:

    λx → x.0.this     ≡    λx → get "this" (get 0 x)
  
    (λx → x.0.this) [{this: 4}]    ≡    4
  

  Additionally, .0 is sugar for (|x| x.0). This composes sanely:

    .0.1.2  ≡  λx → get 2 (get 1 (get 0 x))
  

  Note that this syntax is only available if the to-be-indexed-thing is a variable.

    [1, 2, 3].0     # Parse error!
  

• Instead of manually composing functions, | may be used instead;

    (get 0 | λx → { x.id: x.name }) [{id: 42, name: "Arthur"}, 4]
      ≡  { 42: Arthur }
  

• Lambdas can be written taking multiple arguments, in which case they are automatically curried:

    \x y -> x    ≡    \x -> \y -> x    ≡    |x, y| x
  

• A shadowed variable may be accessed using its De Bruijn index:

    λ> (λx → λx → x@2) 1 2
    variable not in scope: x@2
    λ> (λx → λx → x@1) 1 2
    1
    λ> (λx → λx → x@0) 1 2
    2
    λ> (λx → λx → x  ) 1 2
    2
  

• Various binary operators can be written in pettier/alternative ways:

  • Multiplication: *, ·
  • Division: /, ÷
  • Equality: =, ==
  • Non-equality: !=, /=, ≠
  • Less-or-equal: <=, ≤
  • Bigger-or-equal: >=, ≥

The type system

rq is a statically typed language with subtyping.¹ The type system looks as follows:

• Primitive types: JSON, Num, and Str, where Num, Str ≤ JSON.

• Universal quantification: ∀a. «Type» or forall a. «Type». The type variable a can be any valid identifier (that is not already a primitive type)

• Function types: «Type» → «Type» or «Type» -> «Type». Function types are contravariant in their first, and covariant in their second argument.

• List types: [«Type»]. Lists are covariant.

Standard library

• Numerical operators: n

  (+)  : JSON → JSON → JSON  -- Also works for string concatenation
  (-)  : JSON → JSON → JSON
  (*)  : JSON → JSON → JSON
  (/)  : JSON → JSON → JSON

• Comparisons:

  Essentially, everything that is not false or null is considered truthy.

  (=)  : JSON → JSON → JSON
  (!=) : JSON → JSON → JSON
  (<)  : JSON → JSON → JSON
  (<=) : JSON → JSON → JSON
  (>)  : JSON → JSON → JSON
  (>=) : JSON → JSON → JSON

• Higher order functions:

  -- `map f xs` applies `f` to every "value" in `xs`, which may be an
  -- array (in which case value means element), or an object (in which
  -- case it really means value).
  map : (JSON → JSON) → JSON → JSON
  
  -- Like map, `filter p xs` applies `p` to every value of `xs`.
  -- Keep the elements for which the predicate returns truthy.
  filter : (JSON → JSON) → JSON → JSON
  
  -- Left-associative fold over an array or (values of an) object; e.g.,
  --
  --   foldl f init [x₁, x₂, …, xₙ]  ≡  f(f(…f(init, x₁), …), xₙ)
  --
  foldl : (JSON → JSON → JSON) → JSON → JSON → JSON

• Misc

  -- The identity function.
  id    : ∀a. a → a,
  
  -- Return the first argument
  const : ∀a. ∀b. a → b → a
  
  -- `get i x` gets the i'th thing out of x. I should be (evaluate to) a
  -- number or a string, with x evaluating to array or object, respectively.
  get   : JSON → JSON → JSON
  
  -- `set i x` coll sets the i'th thing in coll to x, where i
  -- should be a number or a string, and coll should evaluate to
  -- an array or object.
  set   : JSON → JSON → JSON → JSON

REPL

A REPL is provided for getting familiar with the language; call rq with a repl positional argument:

$ rq repl
λ>

By default, expressions will first be type-checked, and then evaluated as far as they can:

λ> 1 + 2
3

λ> |x| x
λx'. x'

λ> \x -> x x
Occurs check: can't construct infinite type: b ≡ b → c

λ> \x -> ids x
variable not in scope: ids

λ> (get 0 | λx → { x.id: x.name }) [{id: 42, name: "Arthur"}, 4]
{ 42: Arthur }

Additionally, the following keywords are available:

• Pretty-print the expression given (this just runs the parser, followed by the pretty-printer): :e

  λ> :e \x -> x x
  λx. (x x)
  
  λ> :e \x -> get 0 x + 3 * 5 - 7
  λx. (- (+ (get 0 x) (· 3 5)) 7)
  

• Type-check an expression, and print the type: :t

  λ> :t \f -> \g -> \x -> f x (g x)
  (a → b → c) → (a → b) → a → c
  
  λ> :t \x -> get 0 x + 3 * 5 - 7
  JSON → JSON
  
  λ> :t map
  (JSON → JSON) → JSON → JSON
  
  λ> :t \x -> x x
  Occurs check: can't construct infinite type: b ≡ b → c
  

• Get information on a builtin function with :i:

  λ> :i <
  e < e' checks whether e is less than e'
  
  λ> :i map
  map f xs applies f to every value in xs, which may be an
  array (in which case »value« means element) or an object
  (in which case it really means value).
  

• To list all builtin functions, use :l:

  λ> :i map
  +	Add two number, or concatenate two strings.
  -	Subtract two numbers.
  <	e < e' checks whether e is less than e'
  … and so on …
  

• Debugging: :d

  λ> :d \x -> x 4 "flurble"
  Lam("x", App(App(Var("x"), Const(Num(OrdF64(4.0)))), Const(String("flurble"))))
  

  • Prettier, yet more verbose, output: :dp

      λ> :dp \x -> x x
      Lam(
          "x",
          App(
              Var(
                  "x",
              ),
              Var(
                  "x",
              ),
          ),
      )
    

Additional command line flags

For no reason at all, there are some additional command line flags; they ostensibly have nothing to do with rq's main functionality:

• --flatten (-f): Flatten the given JSON into a list. Inspired by gron.

Footnotes

1. This will be indicated by SubType ≤ T. ↩