[Hugs-users] OpTrex

[Anthony Clayden]

It's always seemed to me that field labels in records are more
constructor-like than term-like. And pattern-matching on them is more like
using them as (de-)constructors.

But H98 records create a selector function named for the field; and because
it's a function it must be term-like.

TRex field labels don't suffer this restriction: you don't get a selector
function; if you want a select by label from a TRex row, use the `#label
row` syntax.

Then I wondered: TRex labels appear only in a handful of syntax
productions. How hard would it be to allow them to look like constructors?
The answer is it's dead easy; you need only to change the syntax (in yacc
plus a tweak to the lexer); you don't need to change any of the code for
building/matching/typing rows. For now, I'm allowing labels to start either
upper or lower case.

Some example code here
https://github.com/ghc-proposals/ghc-proposals/pull/160#issuecomment-420272180

In particular, there's an example defining a Lens `bar` over any Trex row
containing a label `Bar`. (The main purpose of that definition is to
demonstrate every syntactic construct in which TRex allows labels, so it
could be terser.) Note that unlike Lenses over H98 style records, this
doesn't need an overloading for each different datatype containing a label
of some name.

My jazzy name for such Lense is 'OpTRex', as imagined here
https://github.com/ghc-proposals/ghc-proposals/pull/158#issuecomment-414060490


AntC
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