= Polymorphic record patterns ('''''speculative''''' future) =

John Meacham has requested the ability to put polymorphic record syntax in a pattern. http://www.haskell.org/pipermail/glasgow-haskell-users/2012-February/021918.html

Without trying to match his request exactly, here's a possible desugarring within the DORF framework. (I am __not__ suggesting this gets implemented until we've more experience with DORF.)

Syntax like this in a pattern:
{{{
case r of {
    r{customer_id, firstName, lastName } -> ... customer_id ...
    }
}}}
Note:
 * there's a lower-case name prefix to the `{ ... }` body, so this is __not__ a data constructor
 * this is an irrefutable name binding, not really a pattern
 * (That is, if `r` doesn't happen to have all those fields, we'll just fail to compile, we won't try to go on to the next case branch.)
 * This example is using '''-XNamedFieldPuns''', we could also use the `{ blah = foo }` syntax John gives.

The obvious way to desugar:
{{{
let { customer_id = customer_id r; ...
  in { ... customer_id ...}
}}}
won't work because we get name capture inside the let declarations. (The dangers of punning!) So it'll have to be:
{{{
let { customer_id_via = customer_id r; ... }
  in { let { customer_id = customer_id_via; ... }
         in { ... customer_id ...}
}}}
    [Implementor's note: can we improve this?]
Within the body we can indeed use polymorphic record update syntax to reconstitute the record:
{{{
... r{ lastName = map toUpper lastName } ...
}}}
Note that since there's no data constructor, this will continue `r`'s existing data constructor.