assert()

Checks for a condition. If the condition is false, outputs error messages and fails the query.

Syntax

assert(condition,message)

Parameters

Name	Type	Required	Description
condition	bool	✓	The conditional expression to evaluate. The condition must be evaluated to constant during the query analysis phase.
message	string	✓	The message used if assertion is evaluated to false.

Note

condition must be evaluated to constant during the query analysis phase. In other words, it can be constructed from other expressions referencing constants, and can't be bound to row-context.

Returns

Returns true if the condition is true. Raises a semantic error if the condition is evaluated to false.

Examples

The following query defines a function checkLength() that checks input string length, and uses assert to validate input length parameter (checks that it's greater than zero).

Run the query

let checkLength = (len:long, s:string)
{
    assert(len > 0, "Length must be greater than zero") and
    strlen(s) > len
};
datatable(input:string)
[
    '123',
    '4567'
]
| where checkLength(len=long(-1), input)

Running this query yields an error: assert() has failed with message: 'Length must be greater than zero'

Example of running with valid len input:

Run the query

let checkLength = (len:long, s:string)
{
    assert(len > 0, "Length must be greater than zero") and strlen(s) > len
};
datatable(input:string)
[
    '123',
    '4567'
]
| where checkLength(len=3, input)

Output

input
4567