[Haskell-cafe] Field types

[Anthony Clayden]

Report section 4.2.1

"A `data` declaration may use the same field label in multiple constructors
as long as the typing of the field is the same in all cases after type
synonym expansion."

> In that hypothetical context, the field name wouldn't be usable as a
> function—at least without future Dependent Haskell.

TRex manages to support same label different field type in different records.

The record's type includes the field labels, as a distinct Kind.
`#lab` is the syntax to access a field labelled `lab`.

>    #lab :: a\lab => Rec (lab :: b | a) -> b

`(lab :: b | a)` is of Kind Row, means a row with label `lab` at type
`b`; `| a` captures any other fields in the record; `Rec( )` is a
magic type constructor that makes Rows into Kind `*`. Context `a\lab`
means `a` must not include label `lab`.

Hugs.Trex> let myTuplePair = (( lab = 5 :: Int, lab2 = True), (lab = 'c',
lab3 = Just 7))
>                                    in (#lab $ fst myTuplePair, #lab $ snd
myTuplePair)

>    (5,'c')


On Mon, Sep 20, 2021, 1:19 PM Tom Ellis <tom-lists-haskell-cafe-2017
at jaguarpaw.co.uk
<http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe>>
wrote:

>* On Mon, Sep 20, 2021 at 01:13:43PM -0400, David Feuer wrote:
*>* > Does one field name for one datatype always refer to a field with the
*>* same
*>* > type? Or is there some wacky extension that would allow things like
*>* >
*>* > data Foo
*>* >   = Bar { zoom :: Int }
*>* >   | Baz { zoom :: Char }
*>* >
*>* > I'm hoping I don't have to worry about the latter possibility....
*>>* Me too!  Under such circumstances what would the type of
*>* field-as-function be?*
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