runtime-arbitrary

Last updated: 2017-07-28 13:21:56 +0100

Upstream URL: git clone http://chriswarbo.net/git/runtime-arbitrary.git

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Obtain data generators at run time, from instances of Arbitrary.

We define one function with the following (simplified) signature

RuntimeArbitrary.getArbGen :: Typeable a =>gt; proxy a ->gt; [Gen a]

The proxy type constructor can be anything, whatever makes it easiest to provide the argument. For example [] :: [Foo] is a perfectly good argument, as long as Foo is Typeable.

The resulting list will contain the arbitrary method of the Arbitrary type class, if a suitable instance is in scope at initialisation time (see below). If not, an empty list is returned. (Really, this is a Maybe (Gen a) rather than a [Gen a], but it's less verbose to manipulate lists than maybes).

Initialisation is required to find instances, and requires the following line appear somewhere in your module (or one of its imports):

mkIfCxtInstances ''Arbitrary

This is Template Haskell, and uses the IfCxt module from the ifcxt package, so you must have all of those things enabled/imported. This is where the magic happens.

Conceptually, this initialisation line generates, at compile time, a Map of all Arbitrary instances which are in-scope at that point (i.e. available via imports), indexed by type rep. At run time, when we execute a call to getArbGen, it simply looks up the given type rep in this Map, to get the corresponding arbitrary method.

Since Template Haskell cannot (yet) manipulate a module's imports, this need to initialise "manually" seems unavoidable (if runtime-arbitrary did this initialisation for you, only those instances we decide to import would be available; with no extension mechanism).