In haskell, does s k v ~ s1 k1 v1, where s :: * -> * -> *, imply that k ~ k1, v ~ v1, or s ~ s1? If not, why doesn't it?
I encountered this when writing some experimental code, a small piece of which is:
newtype Article = Article String
newtype ArticleId = ArticleId Int
newtype Comment = Comment String
newtype CommentId = CommentId Int
data TableName k v where
Articles :: TableName ArticleId Article
Comments :: TableName CommentId Comment
data CRUD k v r where
Create :: v -> CRUD k v k
Read :: k -> CRUD k v (Maybe v)
data Operation t r where
Operation :: s k v -> CRUD k v r -> Operation (s k v) r
operatesOn :: (Eq (s k v)) => s k v -> Operation (s k v) r -> Bool
operatesOn tableName1 (Operation tableName2 _) = tableName1 == tableName2
Which doesn't compile due to the following error:
Could not deduce (v1 ~ v)
from the context (Eq (s k v))
bound by the type signature for
operatesOn :: Eq (s k v) => s k v -> Operation (s k v) r -> Bool
or from (s k v ~ s1 k1 v1)
bound by a pattern with constructor
Operation :: forall r (s :: * -> * -> *) k v.
s k v -> CRUD k v r -> Operation (s k v) r,
in an equation for `operatesOn'
`v1' is a rigid type variable bound by
a pattern with constructor
Operation :: forall r (s :: * -> * -> *) k v.
s k v -> CRUD k v r -> Operation (s k v) r,
in an equation for `operatesOn'
at src\Example\Error.hs:44:24
`v' is a rigid type variable bound by
the type signature for
operatesOn :: Eq (s k v) => s k v -> Operation (s k v) r -> Bool
at src\Example\Error.hs:43:15
Expected type: s k v
Actual type: s1 k1 v1
In the second argument of `(==)', namely `tableName2'
In the expression: tableName1 == tableName2
In an equation for `operatesOn':
operatesOn tableName1 (Operation tableName2 _)
= tableName1 == tableName2
If I change the definition of Operation to
data Operation t r where
Operation :: (t ~ s k v) => t -> CRUD k v r -> Operation t r
The operatesOn function will compile, similar to how it did for Maybe
The (t ~ s k v) => constraint(?) will still reject incorrect programs like:
doesntCompile = Operation Article $ Create $ Comment "Yo"
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