Expressions

How expression are evaluated, compared, etc.

Expressions are components that can be evaluated to a value.

Example:

1 + 1 # expression
-> 2  # result

An expression can be found in two forms:

  • unary: meaning it contains only one component
  • binary: meaning it contains two expressions, or operands, and one operator. i.e. <expr> <operator> <expr>

Example:

/* Unary expressions */
1
name
"foo"

/* Binary expressions */
age >= 18
1 AND 0

Here is a list of expressions supported by Genji.

Literal expressions

Any literal evaluates to the closest compatible type.

Strings

Strings are evaluated to the TEXT type, which are utf-8 encoded.

Integers

Integers are evaluated into the INTEGER type.

If an integer is bigger than the maximum value of a 64 bit integer or smaller than the minimum 64 bit integer value, it will be evaluated as a DOUBLE.

Floats

Floats are evaluated into the DOUBLE type.

Booleans

Booleans are evaluated into the BOOL type.

Arrays

Arrays are evaluated into the ARRAY type.

Documents

Documents are evaluated into the DOCUMENT type.

Paths

Paths are evaluated into the value they refer to. They are used to select a value from a document. Their type will depend on the type of the value extracted from the document.

Given the following document:

{
    "recipes": 10,
    "cooking-time": {
        "eggs": [3, 6, 9]
    },
}

Here are examples on how dot notation is evaluated:

recipes
-> 10

`cooking-time`
->  {
        "eggs": [
            3,
            6,
            9
        ]
    }

`cooking-time`.eggs[2]
-> 9

Operators

Genji provides a list of operators that can be used to compute operations with expressions. Operators are binary expressions, meaning they always take exactly two operands. It is possible though to combine multiple operators to create an evaluation tree.

Logical operators

Logical operators are operators that return a boolean under certain conditions.

Name Description
AND Evaluates to true if both operands are truthy
OR Evaluates to true if either the left operand or the right are truthy

An expression is truthy if it evaluates to a non zero-value of its type.

Comparison operators

These operators are used to compare values and evaluate to a boolean.

Name Description
= Evaluates to true if operands are equal, otherwise returns false
!= Evaluates to true if operands are not equal, otherwise returns false
> Evaluates to true if the left-side expression is greater than the right-side expression, otherwise returns false
>= Evaluates to true if the left-side expression is greater than or equal to the right-side expression, otherwise returns false
< Evaluates to true if the left-side expression is less than the right-side expression, otherwise returns false
<= Evaluates to true if the left-side expression is less than or equal to the right-side expression, otherwise returns false
IN Evaluates to true if the left-side expression is equal to one of the values of the right-side array
NOT IN Evaluates to false if the left-side expression is equal to one of the values of the right-side array
IS Has the same behaviour as = except that it returns true if both operands are NULL
IS NOT Has the same behaviour as != except that it supports comparing with NULL

Examples:

1 = 1
-> true

1 > 2.5
-> false

3 IN [1, 2, 3]
-> true

Conversion during comparison

Prior to comparison, an implicit conversion is operated for the operands to be of the same type. Not all types can be compared together. When two incompatible types are compared, the comparison always returns false, except if one of the operands is NULL, in that case it returns NULL.

Example:

1 > "hello"
-> false
1 < "hello"
-> false

The comparison follows a list of rules that are executed in order:

  • If one of the operands is NULL, return NULL.
  • If both operands are documents, use the Comparing documents rule
  • If both operands are arrays, use the Comparing arrays rule
  • If both operands are numbers (INTEGER or DOUBLE), cast the integer to DOUBLE then compare them together.
  • If both operands have the same type, compare them together.

In any other case, return false.

Comparing documents

The fields of each document are sorted, then they are compared one by one, until they are found not equal. The comparison is then determined by the result of the comparison between these two values. If both keys are equal, compare the values.

{a: 1, b: 2} = {b: 2, a: 1}
-> true

{} = {}
-> true

{a: 1, b: 3} > {a: 1, b: 2}
-> true

{a: 100} > {aa: 1}
-> false

Comparing arrays

Each elements of both arrays are compared one by one, index by index, until they are found not equal. The comparison is then determined by the result of the comparison between these two values.

[1, 2, 3] > [1, 1 + 1, 1]
-> true

Let’s break down the example above:

  1. Index 0: 1 and 1 are equal, the comparison continues
  2. Index 1: 2 and 1 + 1 are equal, the comparison continues
  3. Index 2: 3 is greater then 1, the comparison stops and the first array is considered greater than the second one

Two empty arrays are considered equal:

[] = []
-> true

The size of arrays doesn’t matter, unless all the elements of the smallest one are equal to the other one. In that case the biggest array is considered greater.

[3] > [1, 100000]
-> true

[1, 2] < [1, 2, 3]
-> true

Arithmetic operators

Name Description
+ Adding two values
- Substracting two values
* Multiplying two values
/ Dividing two values
% Find the remainder after division of one number by another
& Bitwise AND
| Bitwise OR
^ Bitwise XOR

Arithmetic operations are supported only for the following types:

  • INTEGER
  • DOUBLE

Note that INTEGER and DOUBLE types can be calculated together, in that case INTEGER values will be converted to DOUBLE prior the operation.

Any usage of these operators on incompatible types will return NULL.

3 + 3.5
-> 6.5

3 + '1'
-> NULL

The case of NULL

Any arithmetic operation with one of the operand being NULL returns NULL.

NULL + 1
-> NULL

5 * 10 - NULL
-> NULL

Division rules

The division obeys a few rules depending on the types of the operands:

  • Dividing two integers, always results in an integer
  • Dividing by zero, returns NULL

Return type and overflow

The type of the result of an operation doesn’t necessarily match the type of the operands.

  • The result of a DOUBLE operation will always return a DOUBLE
  • The result of an INTEGER operation will return an INTEGER, unless the return value is bigger than the maximum value of 64-bit integer. In that case, the return type will be a DOUBLE

Evaluation tree and precedence

When parsed, an expression is turned into an evaluation tree so it is possible to combine operators to form complex expressions. The order in which these expressions are executed depends on the priority of the operator.

Here is the list of operators ordered by ascending precedence. Operators with higher precedence are executed before the ones with lower precedence

  • OR
  • AND
  • =, !=, <, <=, >, >=
  • +, -, |, ^
  • *, /, %, &

Example:

3 + 4 * 2 > 10 AND 2 - 2 = false
-> true

This expression can be represented as the following tree:

.
└── AND
    ├── >
    │   ├── +
    │   │   ├── 3
    │   │   └── *
    │   │       ├── 4
    │   │       └── 2
    │   └── 10
    └── -
        ├── 2
        └── 2

The deepest branches will be executed first, recursively until reaching the root.


Last modified April 13, 2021: Small fixes (dfa008b)