There are many libraries to write function "takes an record has Foo and Bar and returns something." But writing type of the function is still difficult. I can't write such types using HList or records without reading documents. I think, using has, There's few effort to write such types.
In which manner do you need to read less documentation to write:
] f :: Has Foo r => r -> ...
Instead when using HList:
] f :: HasField Foo record fieldType => ...
HasField only gives projection function (hLookupByLabel), but Has gives projection, injection and modification function. If I want to write a generic function injecting a value into field Foo in a record by HList, I should read documentation more.
I think `has' fits the needs of Haskellers who have the good habit of writing a type of a function before its definition.
What does this mean exactly in terms of the type inference possible?
Probably, yes. it's still fragile due to some reasons e.g. the behavior of UndecidableInstances language extension.
import Data.Has data Foo = Foo; type instance TypeOf Foo = Int data Bar = Bar; type instance TypeOf Bar = Int f r = (Foo ^. r) + (Bar ^. r)
*Main> :t f f :: forall s. (Contains (Labelled Foo Int) s, Contains (Labelled Bar Int) s) => s -> TypeOf Foo -nwn