[ghc-steering-committee] Discussion for #158 "Add `setFild` to `HasField`"

[Iavor Diatchki]

Hello,

let's start the discussion on proposal #158.

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

-- `x` is the name of the field, `r` is the type of the record, `a` is the
type of the field
class HasField x r a | x r -> a where
  hasField :: r -> (a -> r, a)

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.

This seems like a reasonably simple change, that adds new functionality, so
I recommend that we accept the change.

-Iavor
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