Pattern Synonym Signature Confusion

Simon Peyton Jones simonpj at
Thu Oct 1 21:48:24 UTC 2015


What you say sounds reasonable to me.  I certainly don't have a strong opinion that the current design is the "right" one.


| -----Original Message-----
| From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Matthew
| Pickering
| Sent: 01 October 2015 12:23
| To: GHC developers
| Subject: Pattern Synonym Signature Confusion
| I think that the current state of pattern synonym signatures is quite
| confusing, especially regarding the constraints. For those unfamiliar,
| a signature looks like the following,
| pattern ExNumPat :: (Show b) => (Num a, Eq a) => b -> T a
| The first constraint being the "provided constraints" and the second
| the "required constraints".
| My main concern is when only a single constraint is specified then
| these are treated as the provided constraints. The manual gives the
| reason that this is the "more common" choice. It seems that this
| motivation is driven by the original ticket which had a lengthy
| discussion about GADTs. In my experience, the opposite is true, it is
| more common to have required constraints.
| This is true especially in a few common cases such as "pattern Foo =
| 27", where users define pattern synonyms which have (overloaded)
| literals on the RHS. The most general signature for such a pattern is
| "pattern :: () => (Eq a, Num a) => a".
| For this reason, I think it would be better if either
| 1. Only specifying one constraint corresponded to the required constraints
| 2. We required users to specify both sets of constraints, even if the
| provided constraints are empty.
| In terms of breakage, I don't think that pattern synonym signatures
| are widely used yet. In both schemes it is possible to write backwards
| compatible code by writing both sets of constraints.
| I think a lot of the confusion also arises from the unusual form of
| the signature, it is unusual to specify two sets of constraints and I
| suspect users tends to assume that a single set of constraints is
| either provided or required depending on what they want it to mean.
| Forcing the specification of both, forces the user to make the
| distinction earlier rather than trying to decipher error messages.
| Thoughts?
| Matt
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