On Thu, 11 Mar 2021, Richard Eisenberg wrote:
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)
Heck, even just pattern matching on `0` (no view patterns) Requires the
scrutinee to be both `Num` and `Eq`. To see why, ponder the inferred
type of the following function:
```
foo 0 = True
foo _ = False
```
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