Record syntax for pattern synonyms

[Dr. ÉRDI Gergő]

Record field updates via patsyns looks very weird to me (and, as just a
user, it would be unexpected). Can't we do just matchers and builders for
now, and add field updaters as a second step, if there's concensus that
it's a Good Idea?

Bye,
Gergo
On 11 Aug 2015 07:11, "Matthew Pickering" <matthewtpickering at gmail.com>
wrote:

> I was looking at implementing #8582 but before I got too far I thought
> it best to clear up a few design points.
>
> A summary can be found below and a more fleshed out version with some
> examples can be found on the wiki page[1].
>
> My main question is about how best to deal with record updates. Say
> that Foo is a record pattern synonym then how would we expect the
> following program to behave?
>
> ```
> foo a at Foo{..} = a {bar = baz}
> ```
>
> Then say that `pattern Foo{bar} = Just bar`, how should the following
> two programs behave? Is this partiality any different to that caused
> by ordinary use of pattern synonyms? (At least partiality in patterns
> is warned but how comprehensive is the coverage?)
>
> ```
> foo :: Maybe a -> Maybe Int
> foo x = x {bar = 5}
>
> -- error as `bar` unique determines that we need Foo
> bar = Nothing {bar = 5}
> ```
>
> Abandoning record updates seems to make record syntax for pattern
> synonyms far less useful and confusing to users. Is this design how
> others have imagined it? I have cced Gergő who originally implemented
> the extension and created #8582.
>
> Matt
>
> ----
>
> Unidirectional patterns
> * Provide the same ability to match as normal records (RecordWildcards etc)
> * Provide selector functions
>
> Bidirectional patterns
> * Provide the constructor which can be used as normal record constructors
>
> Record Updates - unclear
> * Generalise update syntax to arbitrary expressions?
>
>
> [1]:
> https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms#RecordPatternSynonyms
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