<div dir="ltr">Hello,<div><br></div><div>does anyone else have any input on this proposal? </div><div> </div><div>There has been some discussion on Simon's point about using a type-family instead of a fun-dep. The outcome of the discussion is a little unclear, but here is a very brief summary to the best of my understanding:</div><div> * Either approach works, but with the current GHC implementations both approaches have some pros and some cons.</div><div> * Fun-deps (used in the current design) are a bit more restrictive as they do not produce evidence at the moment</div><div> * There are two ways to use type families instead of fun-deps:</div><div> a) the class has 2 parameters, and the third is computed from them using a type family</div><div> b) the class remains with 3 parameters, but it gets a super-class constraint, where a type-family encodes the functional dependency</div><div> * Either type family encoding leads to types that look more verbose, both in errors and inferred types.</div><div> * There are some ideas about how this might be improved for type families in general (see #16070)</div><div><br></div><div>The choice of type-families/fun-deps is quite orthogonal to the original proposal, which is about adding a way to update records. We were discussing it,</div><div>because it is quite tempting to roll-up multiple interface breaking changes into one.</div><div><br></div><div>In the interest of making progress, my vote would be to accept the proposal as is, and delay switching to type families to a separate proposal,</div><div>which might look better once the improvements in #16070 are figured out.</div><div><br></div><div>What does everyone else think?</div><div><br></div><div>-Iavor</div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr">On Wed, Dec 19, 2018 at 1:04 AM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:12pt">Iavor<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12pt">I’m broadly happy, but I would like us (and the proposers) to discuss the question of using a type family instead of a fundep. See my comment at
<a href="https://github.com/ghc-proposals/ghc-proposals/pull/158#issuecomment-448520004" target="_blank">
https://github.com/ghc-proposals/ghc-proposals/pull/158#issuecomment-448520004</a><u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12pt">Simon<u></u><u></u></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-steering-committee <<a href="mailto:ghc-steering-committee-bounces@haskell.org" target="_blank">ghc-steering-committee-bounces@haskell.org</a>>
<b>On Behalf Of </b>Iavor Diatchki<br>
<b>Sent:</b> 18 December 2018 18:02<br>
<b>To:</b> ghc-steering-committee <<a href="mailto:ghc-steering-committee@haskell.org" target="_blank">ghc-steering-committee@haskell.org</a>><br>
<b>Subject:</b> [ghc-steering-committee] Discussion for #158 "Add `setFild` to `HasField`"<u></u><u></u></span></p>
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Hello,<u></u><u></u></p>
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let's start the discussion on proposal #158.<u></u><u></u></p>
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After some discussion on the pull request the proposal was changed, so that instead of adding another method, it now proposes to remove the existing method `getField`, and add a new method `hasField`, which allows to both access and change the value of a field.
After the proposal the class would look like this<u></u><u></u></p>
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-- `x` is the name of the field, `r` is the type of the record, `a` is the type of the field<u></u><u></u></p>
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class HasField x r a | x r -> a where<u></u><u></u></p>
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hasField :: r -> (a -> r, a)<u></u><u></u></p>
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In addition, we'd provide two functions `getField` and `setField` which are defined in terms of `hasField`. The proposal may break existing code, if it defines manual instances of `HasField`, however, code that just uses the functionality will continue working
as before. `HasField` is relatively new, and there aren't many reasons to define custom instances of it, so it is expected that breaking code would not be a big issue.<u></u><u></u></p>
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This seems like a reasonably simple change, that adds new functionality, so I recommend that we accept the change.<u></u><u></u></p>
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-Iavor <u></u><u></u></p>
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