You're right that these features sit in a similar space. The difference is
that, with a pattern synonym, the required context might be useful. This is
because pattern synonyms can perform computation (via view patterns), and this
computation might plausibly require some class constraint. An easy example:
> pattern Positive :: (Ord a, Num a) => a
> pattern Positive <- ((>0) -> True)
Here, the required context is helpful. On the other hand, because matching
against a data constructor never does computation, the constraints are never
useful in this way.
Richard
> On Mar 9, 2021, at 7:02 PM, Anthony Clayden <anthony_clay...@clear.net.nz>
> wrote:
>
> I must be slow on the uptake. I've just grokked this equivalence -- or is it?
> Consider
>
> > data Eq a => Set a = NilSet | ConsSet a (Set a) -- from the Language
> > report
> >
> > -- ConsSet :: forall a. Eq a => a -> Set a => Set a -- inferred/per
> > report
> >
> > -- equiv with Pattern syn 'Required' constraint
> > data Set' a = NilSet' | ConsSet' a (Set' a) -- no DT context
> >
> > pattern ConsSetP :: (Eq a) => () => a -> (Set' a) -> (Set' a)
> > pattern ConsSetP x xs = ConsSet' x xs
> >
> > ffP ((ConsSet x xs), (ConsSetP y ys)) = (x, y)
> >
> > -- ffP :: forall {a} {b}. (Eq a, Eq b) => (Set a, Set' b) -> (a, b) --
> > inferred
>
> The signature decl for `ConsSetP` explicitly gives both the Required `(Eq a)
> =>` and Provided `() =>` constraints, but the Provided could be omitted,
> because it's empty. I get the same signature for both `ConsSetP` as `ConsSet`
> with the DT Context. Or is there some subtle difference?
>
> This typing effect is what got DT Contexts called 'stupid theta' and
> deprecated/removed from the language standard. ("widely considered a
> mis-feature", as GHC is keen to tell me.) If there's no difference, why
> re-introduce the feature for Patterns? That is, why go to the bother of the
> double-context business, which looks weird, and behaves counter to usual
> signatures:
>
> > foo :: (Eq a) => (Show a) => a -> a
> > -- foo :: forall {a}. (Eq a, Show a) => a -> a -- inferred
>
> There is a slight difference possible with Pattern synonyms, compare:
>
> > pattern NilSetP :: (Eq a) => () => (Set' a)
> > pattern NilSetP = NilSet'
> >
> > -- NilSetP :: forall {a}. Eq a => Set' a -- inferred
> > -- NilSet :: forall {a}. => Set a --
> > inferred/per report
>
> Using `NilSetP` somewhere needs giving an explicit signature/otherwise your
> types are ambiguous; but arguably that's a better discipline than using
> `NilSet` and allowing a Set with non-comparable element types.
>
> AntC
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