如何在GHC中使用像`HasDynFlags m`这样的类型类

Question

While playing with GHC code base, I find a typeclass named HasDynFlags :

class HasDynFlags m where
    getDynFlags :: m DynFlags

Although the typeclass name looks self-explanatory, I couldn't find other constraints in the typeclass definition that says m has to be Monad or at least Functor so we can get access to that value.

However, most use of it I find in the code base is inside a do-notation, eg dynFlag <- getDynFlags where m is further constrainted to be an instance of Monad .

My questions are:

• For HasDynFlags m , does m have to be at least Functor to make this typeclass useful?
• If the answer to the first question is no, then how are we supposed to get access to a value of DynFlags given getDynFlags :: m DynFlags , without any further knowledge about m ?

Answer 1

According to the class definition,

class HasDynFlags m where
    getDynFlags :: m DynFlags

m is satisfied by kind (* -> *) . The kind (* -> *) is implied by the type m DynFlags , which demonstrates that m is a type constructor taking exactly one type parameter.

There are no further constraints on m here. Specifically, the resulting type needn't be a Functor (or Monad), although given common naming conventions for type variables in Haskell, there's a good chance Monad is the motivating case.

EDIT: To answer the second question, the Functor or Monad class constraints we expect are introduced in more specific contexts. For example, consider the type,

(HasDynFlags m, Monad m) => m DynFlags

I think that's all there is to it.