> each of which requires some deep knowledge of type theory to understand how it affects the language
This is simply false. Quite a few of them are not much more than syntax sugars, e.g. with LambdaCase I can write
eitherToMaybe = \case
Left _ -> Nothing
Right x -> Just x
instead of
eitherToMaybe var = case var of
Left _ -> Nothing
Right x -> Just x
With OverloadedStrings I can write "foo" instead of (T.pack "foo") or whatever and it picks the right "pack" function based on the inferred type.
OverloadedLists does the same for listy things, e.g. now [1] can be a literal vector and you don't have to say V.fromList [1].
TupleSections lets you write (1,) when you want a function that tuples something with 1 (so (1,) applied to 2 becomes (1,2)).
NamedFieldPuns lets you write
foo x = let myField = x + 1 in MyType{myField}
instead of
foo x = let thing = x + 1 in MyType{myField = thing}
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And then there are a bunch of "Deriving…" extensions that don't require any knowledge either, you just enable the pragma, and it lets you say e.g.
data X … deriving (FromJSON)
instead of having to explicitly implement the FromJSON instance by hand. I have used Deriving a bunch and have no idea how they work :-)
Looking through my main project, the only other extension I use on a regular basis is ScopedTypeVariables. That one's a bit more difficult to explain since you have to have some idea what the words "scope" and "type variable" mean, but basically lets you refer to a type variable from the outer function signature in the type signature of a where-clause. Normally, with
foo :: Maybe a -> Int
… where helper :: Maybe a -> Int
the a's are seen as different types. With ScopedTypeVariables, you can say
foo :: forall a. Maybe a -> Int
… where helper :: Maybe a -> Int
and the a's are the same type. So that lets you have explicit type sigs on helper functions that are restricted to the type of the outer function. (Though I am not an expert on this so maybe I've got it all wrong. But that doesn't stop me from using it to make stuff with.)
