#pretty #string #assertions #numbers #assertion #output #composable #mismatch


A library of composable assertions with pretty output

1 unstable release

0.0.0 Nov 12, 2020




The concepts in Veritas are heavily inspired by Google's truth library.


Leaf Correspondence

A leaf correspondence determines whether a value T corresponds in some way to a Pattern.

There are two main kinds of correspondences:

  • Exact: The T precisely corresponds to the Pattern. Equal and Matches are examples of Exact correspondences.
  • Tolerance: The T corresponds to the Pattern, within some tolerance. CloseTo is an example of a Tolerance correspondence.


A Fact is a key-value pair that can be presented to the user as a part of the explanation for a mismatch.

There are several built-in facts which are designed to help correspondence implementations present useful error messages for the supported reporters.

  • Basic: a single string. For example, when the Present correspondence fails, it produces a Basic fact that renders as "is not present".
  • Key-Value: a static key describing the fact and a value describing the fact in terms of the concrete T
  • Diffable String: a string representation of the T and a string representation of the expected T that should be presented as a rich diff.

Leaf Mismatches

If a Correspondence is not satisfied, it produces a Mismatch. A Mismatch contains a number of Facts that, together, describe the reason for the mismatch.

A Mismatch M2 can depend on another Mismatch M1, which means that M2 doesn't make sense unless M1 is fixed. Normal reporters will likely only present mismatches without dependencies, while verbose reporters could present all mismatches.

Compound Mismatches


Unlike many other expectation systems, mismatches in Strings are modelled as compound mismatches in Veritas. Each Fact about a String specifies a Range into the String that it refers to.

Strings have a number of special kinds of Facts:

  • Ranged Basic: Like a Basic fact, but applied to a specific range in the string
  • Ranged Key-Value: Like a Key-Value fact, but applied to a specific range in the string

Correspondences about String values can use the Diffable String fact to present differences between expected and actual values.

The collection of all Facts about a String creates a set of all problematic ranges in the source string.






Custom Correspondences

Inspired By

This library is inspired by [rspec][rspec], truth, and many other test frameworks in many languages focusing on producing good error output.


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