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Language-Definition.md

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Language Definition

Expr is an expression evaluation language for Go.

Supported Literals

The package supports:

  • strings - single and double quotes (e.g. "hello", 'hello')
  • numbers - e.g. 103, 2.5, .5
  • arrays - e.g. [1, 2, 3]
  • maps - e.g. {foo: "bar"}
  • booleans - true and false
  • nil - nil

Digit separators

Integer literals may contain digit separators to allow digit grouping into more legible forms.

Example:

10_000_000_000

Fields

Struct fields and map elements can be accessed by using the . or the [] syntax.

foo.Field
bar["some-key"]

Functions

Functions may be called using the () syntax.

foo.Method()

Operators

Arithmetic Operators

  • + (addition)
  • - (subtraction)
  • * (multiplication)
  • / (division)
  • % (modulus)
  • ^ or ** (exponent)

Example:

x^2 + y^2

Comparison Operators

  • == (equal)
  • != (not equal)
  • < (less than)
  • > (greater than)
  • <= (less than or equal to)
  • >= (greater than or equal to)

Logical Operators

  • not or !
  • and or &&
  • or or ||

Example:

life < universe || life < everything

String Operators

  • + (concatenation)
  • matches (regex match)
  • contains (string contains)
  • startsWith (has prefix)
  • endsWith (has suffix)

Example:

"hello" matches "h.*"

Membership Operators

  • in (contain)
  • not in (does not contain)

Example:

user.Group in ["human_resources", "marketing"]

"foo" in {foo: 1, bar: 2}

Numeric Operators

  • .. (range)

Example:

user.Age in 18..45

The range is inclusive:

1..3 == [1, 2, 3]

Ternary Operators

  • foo ? 'yes' : 'no'

Example:

user.Age > 30 ? "mature" : "immature"

Builtin functions

  • len (length of array, map or string)
  • all (will return true if all element satisfies the predicate)
  • none (will return true if all element does NOT satisfy the predicate)
  • any (will return true if any element satisfies the predicate)
  • one (will return true if exactly ONE element satisfies the predicate)
  • filter (filter array by the predicate)
  • map (map all items with the closure)
  • count (returns number of elements what satisfies the predicate)

Examples:

Ensure all tweets are less than 280 chars.

all(Tweets, {.Size < 280})

Ensure there is exactly one winner.

one(Participants, {.Winner})

Closures

The closure is an expression that accepts a single argument. To access the argument use the # symbol.

map(0..9, {# / 2})

If the item of array is struct, it is possible to access fields of struct with omitted # symbol (#.Value becomes .Value).

filter(Tweets, {len(.Value) > 280})

Slices

  • array[:] (slice)

Slices can work with arrays or strings.

Example:

Variable array is [1,2,3,4,5].

array[1:5] == [2,3,4] 
array[3:] == [4,5]
array[:4] == [1,2,3]
array[:] == array