ExprML Language

ExprML is a programming language for representing and evaluating expressions in JSON format.

The language specification is defined by a JSON Schema, available here:

• JSON: https://github.com/exprml/exprml-language/blob/main/schema.json
• YAML: https://github.com/exprml/exprml-language/blob/main/schema.yaml

Concept

ExprML realizes the following concepts to make it suitable as an expression language used in configuration files:

• JSON-based:
  • ExprML can represent expressions in JSON (and JSON-compatible YAML).
  • ExprML expressions can be evaluated as JSON values.
  • ExprML expressions can be checked and auto-completed statically using the JSON Schema.
• Embeddable:
  • Interpleters implementing ExprML are available as libraries.
  • JSON Schema of ExpreML can be referenced from exsisting tools and other JSON Schemas.
• Multi-language support
  • Libraries implementing the interpreters are provided in various programming languages.

Interpreters

Libraries

• Go: https://github.com/exprml/exprml-go
• PHP: https://github.com/exprml/exprml-php
• Dart: https://github.com/exprml/exprml_dart
• TypeScript and JavaScript: https://github.com/exprml/exprml-ts
• JVM languages including Kotlin and Java: https://github.com/exprml/exprml-jvm

Console application

https://github.com/exprml/exprml-cli

Syntax

• Literals
  • Boolean
  • Number
  • String
• Composite values
  • Array
  • Object
• Functions and variables
• Operations
  • Unary operations
  • Binary operations
  • Variadic operations
• Conditional branches
• Collection
  • Iteration
  • Element extraction

Literals

Boolean

true
# => true

Number

1089
# => 1089

String

'`ExprML`'
# => "ExprML"

Composite values

Array

arr: [1, 2, 3]
# => [1, 2, 3]

Object

obj:
  id: 1234567
  name: "`ExprML`"
# => {"id": 1234567, "name": "ExprML"}

Functions and variables

eval:
  $hello_func:
    $target: $name_var
where:
  - $name_var: "`ExprML`"
  - $hello_func($target):
      cat: ["`Hello, `", $target, "`!`"]
# => "Hello, ExprML!"

Operations

Unary operations

not: true
# => false

operator	operand	result
len	string, array, object	length of the collection
not	boolean	negation of the operand
flat	array of array	flattened array
floor	number	maximum integer which is less than or equal to the operand
ceil	number	minimum integer which is greater than or equal to the operand
abort	string	It fails the evaluation with a given message.

Binary operations

gte: [ 1234, 567 ]
# => true

operator	operands	result
sub	number	subtraction of the operands
div	number	division of the operands
eq	number, boolean, string, array, object	equality of the operands
neq	number, boolean, string, array, object	negation of the equality of the operands
lt	comparable values, which are numbers, strings, booleans, or arrays of comparable values	whether the left operand is less than the right operand
lte	comparable values	whether the left operand is less than or equal to the right operand
gt	comparable values	whether the left operand is greater than the right operand
gte	comparable values	whether the left operand is greater than or equal to the right operand

Variadic operations

add: [ 1, 2, 3 ]
# => 6

operator	operands	result
add	number	sum of the operands
mul	number	product of the operands
and	boolean	logical conjunction of operands
or	boolean	logical disjunction of the operands
cat	string	concatenation of the operands
min	number	minimum number in the operands
max	number	maximum number is the operands
merge	object	merged object of the operands

Conditional branches

cases:
  - when: false
    then: 1
  - when: true
    then: 2
  - otherwise: 3
# => 2

Collection

Collection is one of string, array, and object.

Iteration

String

for($idx, $elem): "`abc`"
do: $idx
if:
  neq: [$elem, "`b`"]
# => [0, 2]

Array

for($idx, $elem):
  arr: [5, 6, 7]
do: $idx
if:
  neq: [$elem, 6]
# => [0, 2]

Object

for($key, $val):
  obj:
    id: 123
    name: "`ExprML`"
do: $key
if:
  neq: [$val, 123]
# => {"name": "ExprML"}

Element extraction

String

get: 1
from: "`abc`"
# => "b"

Array

get: 1
from: 
  arr: [1, 2, 3]
# => 2

Object

get: "`name`"
from:
  obj:
    id: 123
    name: "`ExprML`"
# => "ExprML"