trait Laws extends AnyRef
Root trait of the law cake.
Defines a wrapper around scalacheck's Properties
(RuleSet), and some default
implementations.
Extend this trait if you want to define a set of laws.
- Source
- Laws.scala
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- class DefaultRuleSet extends RuleSet with HasOneParent
Convenience class for rule sets which may have a parent, but no bases.
- trait HasOneParent extends AnyRef
Convenience trait to mix into subclasses of RuleSet for rule sets which only have one parent.
- trait RuleSet extends AnyRef
This trait abstracts over the various ways how the laws of a type class can depend on the laws of other type classes.
This trait abstracts over the various ways how the laws of a type class can depend on the laws of other type classes. An instance of this trait is called a rule set.
For that matter, we divide type classes into kinds, where the classes of one kind share the number of operations and meaning. For example,
Semigroup
,Monoid
andGroup
all belong to the same kind. On the other hand, their additive variants also belong to a common kind, but to a different one.Users of this trait should extend the outer trait Laws and create specialized subtypes for each kind of type class. (See DefaultRuleSet for an example.)
Consider this example hierarchy:
Semigroup | \ Monoid AdditiveSemigroup | \ | Group AdditiveMonoid \ | AdditiveGroup
They all define their own laws, as well as a couple of parent classes. If we want to check the laws ofAdditiveGroup
, we want to avoid checking properties twice, i.e. do not want to checkMonoid
laws viaGroup
and also viaAdditiveMonoid
.To address this problem, we define the parent in the same kind as parent, and other parents as bases. In this example, the parent of
AdditiveGroup
isAdditiveMonoid
, and its only basis isGroup
. On the other hand, the parent ofGroup
isMonoid
, and it does not have any bases.The set of all properties of a certain class is now defined as union of these sets:
- the properties of the class itself
- recursively, the properties of all its parents (ignoring their bases)
- recursively, the set of all properties of its bases
Looking at our example, that means that
AdditiveGroup
includes theMonoid
law only once, because it is the parent of its basis. The same laws are ignored by its parentAdditiveMonoid
, hence no redundant checks occur.Of course, classes can have multiple parents and multiple (named) bases. The only requirement here is that inside one kind, the identifier of a property is unique, since duplicates are eliminated. To avoid name clashes between different kinds, the names of properties pulled in via a basis are prefixed with the name of the basis.
For better type-safety, parents are only allowed to come from the same outer instance of Laws, whereas bases are allowed to come from anywhere.
| \ | Group AdditiveMonoid \ | AdditiveGroup as a couple of parent classes. If we want to check the laws of
AdditiveGroup
, we want to avoid checking properties twice, i.e. do not want to checkMonoid
laws viaGroup
and also viaAdditiveMonoid
.To address this problem, we define the parent in the same kind as parent, and other parents as bases. In this example, the parent of
AdditiveGroup
isAdditiveMonoid
, and its only basis isGroup
. On the other hand, the parent ofGroup
isMonoid
, and it does not have any bases.The set of all properties of a certain class is now defined as union of these sets:
- the properties of the class itself
- recursively, the properties of all its parents (ignoring their bases)
- recursively, the set of all properties of its bases
Looking at our example, that means that
AdditiveGroup
includes theMonoid
law only once, because it is the parent of its basis. The same laws are ignored by its parentAdditiveMonoid
, hence no redundant checks occur.Of course, classes can have multiple parents and multiple (named) bases. The only requirement here is that inside one kind, the identifier of a property is unique, since duplicates are eliminated. To avoid name clashes between different kinds, the names of properties pulled in via a basis are prefixed with the name of the basis.
For better type-safety, parents are only allowed to come from the same outer instance of Laws, whereas bases are allowed to come from anywhere.
- class SimpleRuleSet extends DefaultRuleSet
Convenience class for rule sets without parents and bases.
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- def emptyRuleSet: RuleSet
Empty rule set.
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