@@ -1,6 +1,14 @@
1−{- |
1+#if __GLASGOW_HASKELL__ >= 806
2+{-# OPTIONS_GHC -Wno-star-is-type #-}
3+#endif
4+#if __GLASGOW_HASKELL__ >= 800
5+{-# OPTIONS_GHC -Wno-unticked-promoted-constructors #-}
6+#endif
7+
8+{-|
29
3−Generic implementation of the 'arbitrary' method. Example usage:
10+This module is a generic implementation of the 'arbitrary' method. Example
11+usage:
412
513@
614data Foo = Foo
@@ -25,48 +33,192 @@ data Foo = Foo
2533
2634The generated 'arbitrary' method is equivalent to
2735
28−@Foo <$> arbitrary <*> arbitrary@.
36+@
37+Foo '<$>' arbitrary '<*>' arbitrary
38+@.
39+
40+It can also handle a recursive types problem. Assuming a type
41+
42+@
43+data R = R R
44+ deriving Generic
45+@
46+
47+there is no instance
48+
49+@
50+instance Arbitrary R where
51+ arbitrary = genericArbitrary
52+ shrink = genericShrink
53+@
54+
55+If you try to compile this you will get a type level error
56+
57+> • R refers to itself in all constructors
58+
59+Which means that there is no finite term for @R@ because it is recursive. But,
60+if you correct the definition of @R@ like this.
61+
62+@
63+data R = R R | F
64+ deriving Generic
65+@
66+
67+Then it will compile. And the @arbitrary@ generated will not hang forever, because
68+it respects the @size@ parameter.
69+
70+There is a limitation of recursion detection:
71+
72+@
73+data R1 = R1 R2
74+ deriving (Eq, Ord, Show, Generic)
75+ deriving anyclass NFData
76+ deriving Arbitrary via (GenericArbitrary R1)
77+
78+data R2 = R2 R1
79+ deriving (Eq, Ord, Show, Generic)
80+ deriving anyclass NFData
81+ deriving Arbitrary via (GenericArbitrary R2)
82+@
83+
84+This code will compile and the @arbitrary@ generated will always hang. Yes,
85+there is a problem with mutually recursive types.
86+
87+Now lets see an example of datatype with parameters
88+
89+@
90+data A a = A a
91+ deriving (Eq, Ord, Show)
92+ deriving anyclass NFData
93+ deriving (Generic)
94+
95+instance (Arbitrary a) => Arbitrary (A a) where
96+ arbitrary = genericArbitrary
97+ shrink = genericShrink
98+@
99+
100+It should work from first glance, but when compile it will throw an error:
101+
102+@
103+ • Could not deduce (Test.QuickCheck.Arbitrary.Generic.GArbitrary
104+ (A a)
105+ (GHC.Generics.D1
106+ ('GHC.Generics.MetaData "A" "ParametersTest" "main" 'False)
107+ (GHC.Generics.C1
108+ ('GHC.Generics.MetaCons "A" 'GHC.Generics.PrefixI 'False)
109+ (GHC.Generics.S1
110+ ('GHC.Generics.MetaSel
111+ 'Nothing
112+ 'GHC.Generics.NoSourceUnpackedness
113+ 'GHC.Generics.NoSourceStrictness
114+ 'GHC.Generics.DecidedLazy)
115+ (GHC.Generics.Rec0 a))))
116+ (TypesDiffer (A a) a))
117+ arising from a use of ‘genericArbitrary’
118+@
119+
120+Here the @TypesDiffer@ is a type familty dealing with recursive types and
121+helping us to eliminate inproper instances. To convince the compiller, that the
122+@a@ parameter is not an @A a@ we must fix the instance with additional constraint
123+
124+@
125+instance (Arg (A a) a, Arbitrary a) => Arbitrary (A a) where
126+ arbitrary = genericArbitrary
127+ shrink = genericShrink
128+@
129+
130+Now everything compiles and works as expected.
29131
30132-}
31133
32134module Test.QuickCheck.Arbitrary.Generic
33− ( GenericArbitrary(..)
135+ ( -- * Main
136+ genericArbitrary
137+#if MIN_VERSION_QuickCheck(2, 14, 0)
138+ , GenericArbitrary(..)
139+#endif
140+ , Arg
141+ -- * Internal
142+ , GArbitrary
143+ , FiniteSum
144+ , FiniteSumElem
145+ , Finite
146+ , AllFieldsFinal
147+ , TypesDiffer
148+ , ArgumentsCount
149+ , SumLen
150+ -- * Reexports
34151 , Arbitrary(..)
35− , genericArbitrary
36152 , genericShrink
37153 ) where
38154
39155import Control.Applicative
40−import Data.Coerce (coerce)
41156import Data.Proxy
42157import Data.Type.Bool
43158import GHC.Generics as G
44159import GHC.TypeLits
160+import Prelude
45161import Test.QuickCheck as QC
162+#if MIN_VERSION_QuickCheck(2, 14, 0)
163+import Data.Coerce (coerce)
46164import Test.QuickCheck.Arbitrary (GSubterms, RecursivelyShrink)
47165
166+
167+
168+-- | Newtype for @DerivingVia@
169+--
170+-- Usage:
171+--
172+-- @
173+-- data Foo = Foo
174+-- { _fooX :: X
175+-- , _fooY :: Y
176+-- } deriving (Generic)
177+-- deriving (Arbitrary) via GenericArbitrary Foo
178+-- @
179+--
180+-- @since 1.0.0
48181newtype GenericArbitrary a = GenericArbitrary { unGenericArbitrary :: a }
49182 deriving (Show, Eq)
50183
51184instance
52185 ( Generic a,
53− GArbitrary a (Rep a) 'True,
186+ GArbitrary a (Rep a) some,
54187 RecursivelyShrink (Rep a),
55188 GSubterms (Rep a) a
56189 ) => Arbitrary (GenericArbitrary a) where
57190 arbitrary = coerce (genericArbitrary :: Gen a)
58191 shrink = coerce (genericShrink :: a -> [a])
192+#endif
193+
194+-- | Constraint helper for types with parameters
195+--
196+-- Usage:
197+--
198+-- @
199+-- data A a = A a
200+-- deriving (Generic)
201+-- instance (Arg (A a) a, Arbitrary a) => Arbitrary (A a) where
202+-- arbitrary = genericArbitrary
203+-- shrink = genericShrink
204+-- @
205+--
206+-- @since 1.0.0
207+
208+type Arg self field = (TypesDiffer self field ~ 'True)
209+
210+type family TypesDiffer a b where
211+ TypesDiffer a a = 'False
212+ TypesDiffer a b = 'True
59213
60214type family AllFieldsFinal self (a :: * -> *) :: Bool where
61215 AllFieldsFinal self U1 = 'True
62216 AllFieldsFinal self (a :*: b) = AllFieldsFinal self a && AllFieldsFinal self b
63− AllFieldsFinal self (M1 S t (K1 R self)) = 'False
64− AllFieldsFinal self (M1 S t (K1 R other)) = 'True
217+ AllFieldsFinal self (M1 S t (K1 R field)) = TypesDiffer self field
65218
66219type family Finite self (a :: * -> *) :: Bool where
67220 Finite self U1 = 'True
68− Finite self (K1 R self) = 'False
69− Finite self (K1 R other) = 'True
221+ Finite self (K1 R field) = TypesDiffer self field
70222 Finite self (a :*: b) = Finite self a && Finite self b
71223 Finite self (M1 D t f) = Finite self f
72224 Finite self (a :+: b) = Finite self a || Finite self b
@@ -94,8 +246,8 @@ class (Finite self a ~ finite) => GArbitrary self a (finite :: Bool) where
94246 gArbitrary :: Proxy self -> QC.Gen (a x)
95247
96248instance
97− ( GArbitrary self (C1 c f) 'True
98− ) => GArbitrary self (D1 t (C1 c f)) 'True where
249+ ( GArbitrary self (M1 C c f) 'True
250+ ) => GArbitrary self (M1 D t (M1 C c f)) 'True where
99251 gArbitrary _ = M1 <$> gArbitrary (Proxy :: Proxy self)
100252
101253-- | The constructor meta information
@@ -103,7 +255,7 @@ instance
103255 ( GArbitrary self f some
104256 , KnownNat (ArgumentsCount f)
105257 , AllFieldsFinal self f ~ some
106− ) => GArbitrary self (C1 c f) some where
258+ ) => GArbitrary self (M1 C c f) some where
107259 gArbitrary _ = M1 <$> scale predNat (gArbitrary (Proxy :: Proxy self))
108260 where
109261 argumentsCount = fromIntegral $ natVal (Proxy :: Proxy (ArgumentsCount f))
@@ -116,14 +268,14 @@ instance GArbitrary self U1 'True where
116268 gArbitrary _ = pure U1
117269
118270-- | Constructor field meta information
119−instance GArbitrary self f some => GArbitrary self (S1 t f) some where
271+instance GArbitrary self f some => GArbitrary self (M1 S t f) some where
120272 gArbitrary _ = M1 <$> gArbitrary (Proxy :: Proxy self)
121273
122274-- | Data of the constructor field
123275instance
124276 ( Arbitrary t
125− , Finite self (Rec0 t) ~ some
126− ) => GArbitrary self (Rec0 t) some where
277+ , Finite self (K1 R t) ~ some
278+ ) => GArbitrary self (K1 R t) some where
127279 gArbitrary _ = K1 <$> arbitrary
128280
129281-- | Product
@@ -139,7 +291,7 @@ instance
139291instance
140292 ( TypeError (ShowType self :<>: Text " refers to itself in all constructors")
141293 , AllFieldsFinal self f ~ 'False
142− ) => GArbitrary self (D1 t (C1 c f)) 'False where
294+ ) => GArbitrary self (M1 D t (M1 C c f)) 'False where
143295 gArbitrary _ = error "Unreachable"
144296#endif
145297
@@ -147,7 +299,7 @@ instance
147299instance
148300 ( FiniteSum self a b af bf
149301 , GArbitrary self (a :+: b) 'True
150− ) => GArbitrary self (D1 t (a :+: b)) 'True where
302+ ) => GArbitrary self (M1 D t (a :+: b)) 'True where
151303 gArbitrary _ = sized $ \s -> M1 <$>
152304 if s > 1
153305 then gArbitrary (Proxy :: Proxy self)
@@ -203,8 +355,8 @@ instance
203355 finiteElem _ = finiteSum (Proxy :: Proxy self)
204356
205357instance
206− ( GArbitrary self (C1 c f) 'True
207− ) => FiniteSumElem self (C1 c f) where
358+ ( GArbitrary self (M1 C c f) 'True
359+ ) => FiniteSumElem self (M1 C c f) where
208360 finiteElem _ = [gArbitrary (Proxy :: Proxy self)]
209361
210362
@@ -212,7 +364,7 @@ instance
212364instance
213365 ( TypeError (ShowType self :<>: Text " refers to itself in all constructors")
214366 , (Finite self a || Finite self b) ~ 'False
215− ) => GArbitrary self (D1 t (a :+: b)) 'False where
367+ ) => GArbitrary self (M1 D t (a :+: b)) 'False where
216368 gArbitrary _ = error "Unreachable"
217369#endif
218370