To better recognise the pattern, let’s get back to the example above. This time we will map the natural language rules with programming language rules.

Now, I hope you’ll never format code like that, but in this case, that formatting serves well our purpose to show how Ruby’s simplicity helps to define complex rules with no effort.

From a high level perspective, we can tell that input data for name is valid only if all the requirements are satisfied. That’s because we used &.

We support four logic operators:

• & (aliased as and) for conjunction
• | (aliased as or) for disjunction
• > (aliased as then) for implication
• ^ (aliased as xor) for exclusive disjunction

Context Of Execution

Please notice that we used & over Ruby’s && keyword. That’s because the context of execution of these validations isn’t a plain lambda, but something richer.

For real world projects, we want to support common scenarios without the need of reinventing the wheel ourselves. Scenarios like password confirmation, size check are already prepackaged with Hanami::Validations.

For this reason, we don’t allow any arbitrary Ruby code to be executed, but only well defined predicates.

Predicates

To meet our needs, Hanami::Validations has an extensive collection of built-in predicates. A predicate is the expression of a business requirement (e.g. size greater than). The chain of several predicates determines if input data is valid or not.

We already met and size?, now let’s introduce the rest of them. They capture common use cases with web forms.

Array

required(:codes) { array? { each { int? } } }

This example checks if codes is an array and if all the elements are integers, whereas the following example checks there are a minimum of 2 elements and all elements are strings.

required(:codes) { array? { min_size?(2) & each { str? } } }

Emptiness

It checks if the given value is empty or not. It is designed to works with strings and collections (array and hash).

Equality

This predicate tests if the input is equal to a given value.

required(:magic_number) { eql?(23) }

Ruby types are respected: 23 (an integer) is only equal to 23, and not to "23" (a string). See Type Safety section.

It checks if the input is not included by a given collection. This collection can be an array, a set, a range or any object that responds to #include?.

required(:genre) { excluded_from?(%w(pop dance)) }

Format

This is a predicate that works with a regular expression to match it against data input.

Greater Than

This predicate works with numbers to check if input is greater than a given threshold.

required(:age) { gt?(18) }

Greater Than Equal

This is an open boundary variation of gt?. It checks if an input is greater than or equal of a given number.

required(:age) { gteq?(19) }

Inclusion

This predicate is the opposite of #exclude?: it verifies if the input is included in the given collection.

required(:genre) { included_in?(%w(rock folk)) }

Less Than

Similarly to #gteq?, this is the open bounded version of #lt?: an input is valid if it’s less than or equal to a number.

required(:age) { lteq?(6) }

Filled

It’s a predicate that ensures data input is filled, that means not nil or blank ("") or empty (in case we expect a collection).

Minimum Size

This verifies that the size of the given input is at least of the specified value.

required(:password) { min_size?(12) }

Maximum Size

This verifies that the size of the given input is at max of the specified value.

required(:name) { max_size?(128) }

None

This verifies if the given input is nil. Blank strings ("") won’t pass this test and return false.

required(:location) { none? }

Size

It checks if the size of input data is: a) exactly the same of a given quantity or b) it falls into a range.

required(:two_factor_auth_code) { size?(6) }     # exact

The check works with strings and collections.

required(:answers) { size?(2) } # only 2 answers are allowed

This predicate works with objects that respond to #size. Until now we have seen strings and arrays being analysed by this validation, but there is another interesting usage: files.

When a user uploads a file, the web server sets an instance of Tempfile, which responds to #size. That means we can validate the weight in bytes of file uploads.