expect-failure

Use expect-failure to evaluate an expression and succeed only if the expressions results in an error. This function enables you to verify that use cases that should failbehave as expected.

Basic syntax

To expect a failure without specifying the error message, use the following syntax:

(expect-failure doc exp)

To expect a failure with a specific error message, use the following syntax:

(expect-failure doc err exp)

Arguments

Use the following arguments when using the expect-failure Pact function.

Argument	Type	Description
doc	string	Specifies the documentation string describing the expected failure.
err	string	Specifies the expected error message (optional).
exp	any	Specifies the expression to evaluate. The expression can be of any Pact type.

Return value

The expect-failure function returns a string indicating the success or failure of the expected failure.

• If the expression exp throws an error, the function returns a string indicating the success of the expected failure, for example, "Expect failure: success: Enforce fails on false".
• If the expression exp does not throw an error, the function itself throws an error, indicating that the expected failure did not occur.

Examples

The following example demonstrates how to use the expect-failure function without specifying the expected error message:

pact> (expect-failure "Enforce fails on false" (enforce false "Expected error"))
"Expect failure: success: Enforce fails on false"

The following example illustrates using the expect-failure function with a specific expected error message:

(expect-failure "mint fails without MINT capability in scope"
    "Managed capability not installed: (marmalade-v2.ledger.MINT"
    (mint (read-msg 'token-id) (read-msg 'account) (read-keyset 'account-guard) 1.0)))

In this example, the doc argument describes the test being performed. The err argument specifies the error message that is returned when the expression is evaluated.