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The Haskell tutorial I'm reading has a section introducing the basics of type kinds, and describes a type class
class Tofu t where
tofu :: j a -> t a j
as having the kind
* -> (* -> *) -> *
I understand that, but when I enter :k Tofu in GHCi, I get
Tofu :: (* -> (* -> *) -> *) -> GHC.Prim.Constraint
What is GHC.Prim.Constraint and why does the kind of Tofu have this form rather than simply * -> (* -> *) -> *?
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The error message itself showing there is a foreign key constraint error, which means you are deleting a parent table where the child table contains the Primary table identifier as a foreign key. To avoid this error, you need to delete child table records first and after that the parent table record.
DEFAULT Constraint − Provides a default value for a column when none is specified. UNIQUE Constraint − Ensures that all values in a column are different. PRIMARY Key − Uniquely identifies each row/record in a database table. FOREIGN Key − Uniquely identifies a row/record in any of the given database table.
Key constraints Keys are the entity set that is used to identify an entity within its entity set uniquely. An entity set can have multiple keys, but out of which one key will be the primary key. A primary key can contain a unique and null value in the relational table.
t is a type parameter of the class Tofu of kind * -> (* -> *) -> * (written t :: * -> (* -> *) -> *).
This is the inferred type of t by GHC because in the absence of -XPolyKinds, GHC tries to default all type parameters to kind *. Thus GHC assumes a has kind * (though nothing in your signature makes this the only choice).
The type constructor (->) has kind * -> * -> *. Since j a appears as a parameter to (->), j a must have kind *. Since GHC has assumed that a has kind *, j is a type that takes something of kind * and returns something of kind *. Thus:
j :: * -> *
Since t is applied to both a and j, t has the kind * -> (* -> *) -> *, because the first argument a has kind * and the second argument j has kind *->*, and the overall type t a j must have kind * since it is also a parameter of the type (->).
Classes are just types taking type parameters (just like data Foo a b), except Foo a b has kind * while Tofu t has kind a special kind Constraint. Thus the kind of Tofu is:
(* -> (* -> *) -> *) -> Constraint
as GHC indicates. Constraint is just the kind given to constraints. In the signature
(Num a) => a -> a -> a
(Num a) is a type of kind Constraint.
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