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# dry-schema 1.13

## Predicate logic

Schema DSL allows you to define validation rules using predicate logic. All common logic operators are supported and you can use them to compose rules. This simple technique is very powerful as it allows you to compose validations in such a way that invalid state will not crash one of your rules. Validation is a process that always depends on specific conditions, in that sense, `dry-schema` schemas have rules that are always conditional, they are executed only if defined conditions are met.

This document explains how rule composition works in terms of predicate logic.

### Conjunction (and)

``````Dry::Schema.Params do
required(:age) { int? & gt?(18) }
end
``````

`:age` rule is successful when both predicates return `true`.

### Disjunction (or)

``````Dry::Schema.Params do
required(:age) { nil? | int? }
end
``````

`:age` rule is successful when either of the predicates, or both return `true`.

### Implication (then)

``````Dry::Schema.Params do
required(:age) { filled? > int? }
end
``````

`:age` rule is successful when `filled?` returns `false`, or when both predicates return `true`.

Optional keys are defined using `implication`, that's why a missing key will not cause its rules to be applied and the whole key rule will be successful

### Exclusive Disjunction (xor)

``````Dry::Schema.Params do
required(:status).value(:integer) { even? ^ lt?(0) }
end
``````

`:status` is valid if it's either an even integer, or it's value is less than `0`.

### Operator Aliases

Logic operators are actually aliases, use full method names at your own convenience:

• `and` => `&`
• `or` => `|`
• `then` => `>`
• `xor` => `^`

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