Eq package:hedgehog

The Eq class defines equality (==) and inequality (/=). All the basic datatypes exported by the Prelude are instances of Eq, and Eq may be derived for any datatype whose constituents are also instances of Eq. The Haskell Report defines no laws for Eq. However, instances are encouraged to follow these properties:

• Reflexivity x == x = True
• Symmetry x == y = y == x
• Transitivity if x == y && y == z = True, then x == z = True
• Extensionality if x == y = True and f is a function whose return type is an instance of Eq, then f x == f y = True
• Negation x /= y = not (x == y)

Minimal complete definition: either == or /=.

Lift the standard (==) function through the type constructor.

A pre-condition for a command that must be verified before the command can be executed. This is mainly used during shrinking to ensure that it is still OK to run a command despite the fact that some previously executed commands may have been removed from the sequence.

A sequence of actions to execute.

Check a group of properties sequentially. Using Template Haskell for property discovery:

tests :: IO Bool
tests =
checkSequential $$(discover)

With manually specified properties:

tests :: IO Bool
tests =
checkSequential $ Group "Test.Example" [
("prop_reverse", prop_reverse)
]

Executes a list of actions sequentially, verifying that all post-conditions are met and no exceptions are thrown. To generate a sequence of actions to execute, see the sequential combinator in the Hedgehog.Gen module.

The sequence of actions.

Uses a weighted distribution to randomly select one of the generators in the list. This generator shrinks towards the first generator in the list. The input list must be non-empty.

Generates a seq using a Range to determine the length.

Generates a sequence of actions from an initial model state and set of commands.

Generates a random subsequence of a list. For example:

Gen.print (Gen.subsequence [1..5])

=== Outcome ===
[1,2,4]
=== Shrinks ===
[]
[2,4]
[1,4]
[1,2]

Generates a random permutation of a sequence. This shrinks towards the order of the sequence being identical to the input sequence.

Evaluate each monadic action in the structure from left to right, and collect the results. For a version that ignores the results see sequence_.

Examples

Basic usage: The first two examples are instances where the input and and output of sequence are isomorphic.

>>> sequence $ Right [1,2,3,4]
[Right 1,Right 2,Right 3,Right 4]

>>> sequence $ [Right 1,Right 2,Right 3,Right 4]
Right [1,2,3,4]

The following examples demonstrate short circuit behavior for sequence.

>>> sequence $ Left [1,2,3,4]
Left [1,2,3,4]

>>> sequence $ [Left 0, Right 1,Right 2,Right 3,Right 4]
Left 0

Evaluate each action in the structure from left to right, and collect the results. For a version that ignores the results see sequenceA_.

Examples

Basic usage: For the first two examples we show sequenceA fully evaluating a a structure and collecting the results.

>>> sequenceA [Just 1, Just 2, Just 3]
Just [1,2,3]

>>> sequenceA [Right 1, Right 2, Right 3]
Right [1,2,3]

The next two example show Nothing and Just will short circuit the resulting structure if present in the input. For more context, check the Traversable instances for Either and Maybe.

>>> sequenceA [Just 1, Just 2, Just 3, Nothing]
Nothing

>>> sequenceA [Right 1, Right 2, Right 3, Left 4]
Left 4

Evaluate each monadic action in the structure from left to right, and ignore the results. For a version that doesn't ignore the results see sequence. sequence_ is just like sequenceA_, but specialised to monadic actions.