Thanks Gergö, I do find Richard's tendency to use ViewPattern examples distracts me from understanding the point. That arrow-to-nowhere or arrow-to-the-wrong-value syntax is nausea-inducing. Your "instructive" example is really nothing to do with PatSyns, just ordinary Haskell numeric patterns: the Language Report (section 3.17.2 point 7., same wording in 2010 and '98) says a numeric literal in a pattern gets desugared to an `(==)` test against `fromInteger <something>`. So that explains where the constraints come from. (Annoyingly, the <something> you can't express in legit Haskell source.) Avoiding distractions is why my o.p. used an `Ord` constraint, also to follow SPJ's 1999 example. And the canonical example in those old discussions (like Hughes' 1999 paper) is an `Ord` constraint for elements of a `Set` structured as a BST. With your "polymorphic literal" how do I follow the Language Report to desugar? I'd rather be able to write Richard's example using an as-Pattern like one of these:

pattern x@Is3 | x == fromInteger (#3#) = x -- where #3# is the inexpressible <something> pattern x@3@Is3 = x

(It's not that I'm "new to patsyn typing"; it's more that patsyn typing and design keeps falling short of what I hope from such a promising feature. So I always want to check I'm not mis-understanding. Specifically, the design for Pattern Synonyms was a lost opportunity to expunge ViewPatterns entirely.) AntC On Fri, 7 Jan 2022 at 09:43, Gergő Érdi wrote:

Fwiw, a less contrived, and much more relatable, version of Richard's example would be

pattern Is3 :: (Num a, Eq a) => a -- only a Required constraint pattern Is3 = 3 -- a polymorphic literal!

I think it can be quite instructive for people new to patsyn typing to work out why this is exactly the same as the one in Richard's email!

On Thu, Jan 6, 2022, 21:11 Richard Eisenberg wrote:

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