Safe Haskell | None |
---|---|

Language | Haskell2010 |

Generic generation of random test cases.

This module contains a generic version of `arbitrary`

from the
Test.Quickcheck library, using `generics-sop`

.

# Documentation

garbitrary :: forall a. (Generic a, All2 Arbitrary (Code a)) => Gen a Source #

Generic generation of random test cases.

This function is a proof-of-concept implementation of a generic
`arbitrary`

that can be used to instantiate the `Arbitrary`

class
in `QuickCheck`

.

If you want to use it on a datatype `T`

for which you have a
`Generic`

instance, you can say:

instance Arbitrary T where arbitrary = garbitrary

Note that currently no attempts are being made to generate arbitrary values of a particular size, and it is possible that this function diverges for recursive structures.

# Re-exports

Random generation and shrinking of values.

QuickCheck provides `Arbitrary`

instances for most types in `base`

,
except those which incur extra dependencies.
For a wider range of `Arbitrary`

instances see the
quickcheck-instances
package.

A generator for values of the given type.

It is worth spending time thinking about what sort of test data
you want - good generators are often the difference between
finding bugs and not finding them. You can use `sample`

,
`label`

and `classify`

to check the quality of your test data.

There is no generic `arbitrary`

implementation included because we don't
know how to make a high-quality one. If you want one, consider using the
testing-feat or
generic-random packages.

The QuickCheck manual goes into detail on how to write good generators. Make sure to look at it, especially if your type is recursive!

Produces a (possibly) empty list of all the possible immediate shrinks of the given value.

The default implementation returns the empty list, so will not try to
shrink the value. If your data type has no special invariants, you can
enable shrinking by defining `shrink = `

, but by customising
the behaviour of `genericShrink`

`shrink`

you can often get simpler counterexamples.

Most implementations of `shrink`

should try at least three things:

- Shrink a term to any of its immediate subterms.
You can use
`subterms`

to do this. - Recursively apply
`shrink`

to all immediate subterms. You can use`recursivelyShrink`

to do this. - Type-specific shrinkings such as replacing a constructor by a simpler constructor.

For example, suppose we have the following implementation of binary trees:

data Tree a = Nil | Branch a (Tree a) (Tree a)

We can then define `shrink`

as follows:

shrink Nil = [] shrink (Branch x l r) = -- shrink Branch to Nil [Nil] ++ -- shrink to subterms [l, r] ++ -- recursively shrink subterms [Branch x' l' r' | (x', l', r') <- shrink (x, l, r)]

There are a couple of subtleties here:

- QuickCheck tries the shrinking candidates in the order they
appear in the list, so we put more aggressive shrinking steps
(such as replacing the whole tree by
`Nil`

) before smaller ones (such as recursively shrinking the subtrees). - It is tempting to write the last line as
`[Branch x' l' r' | x' <- shrink x, l' <- shrink l, r' <- shrink r]`

but this is the*wrong thing*! It will force QuickCheck to shrink`x`

,`l`

and`r`

in tandem, and shrinking will stop once*one*of the three is fully shrunk.

There is a fair bit of boilerplate in the code above.
We can avoid it with the help of some generic functions.
The function `genericShrink`

tries shrinking a term to all of its
subterms and, failing that, recursively shrinks the subterms.
Using it, we can define `shrink`

as:

shrink x = shrinkToNil x ++ genericShrink x where shrinkToNil Nil = [] shrinkToNil (Branch _ l r) = [Nil]

`genericShrink`

is a combination of `subterms`

, which shrinks
a term to any of its subterms, and `recursivelyShrink`

, which shrinks
all subterms of a term. These may be useful if you need a bit more
control over shrinking than `genericShrink`

gives you.

A final gotcha: we cannot define `shrink`

as simply

as this shrinks `shrink`

x = Nil:`genericShrink`

x`Nil`

to `Nil`

, and shrinking will go into an
infinite loop.

If all this leaves you bewildered, you might try

to begin with,
after deriving `shrink`

= `genericShrink`

`Generic`

for your type. However, if your data type has any
special invariants, you will need to check that `genericShrink`

can't break those invariants.