Matthew Pickering authored 9 years ago and Ben Gamari committed 9 years ago

This patch implements an extension to pattern synonyms which allows user
to specify pattern synonyms using record syntax. Doing so generates
appropriate selectors and update functions.

=== Interaction with Duplicate Record Fields ===

The implementation given here isn't quite as general as it could be with
respect to the recently-introduced `DuplicateRecordFields` extension.

Consider the following module:

    {-# LANGUAGE DuplicateRecordFields #-}
    {-# LANGUAGE PatternSynonyms #-}

    module Main where

    pattern S{a, b} = (a, b)
    pattern T{a}    = Just a

    main = do
      print S{ a = "fst", b = "snd" }
      print T{ a = "a" }

In principle, this ought to work, because there is no ambiguity. But at
the moment it leads to a "multiple declarations of a" error. The problem
is that pattern synonym record selectors don't do the same name mangling
as normal datatypes when DuplicateRecordFields is enabled. They could,
but this would require some work to track the field label and selector
name separately.

In particular, we currently represent datatype selectors in the third
component of AvailTC, but pattern synonym selectors are just represented
as Avails (because they don't have a corresponding type constructor).
Moreover, the GlobalRdrElt for a selector currently requires it to have
a parent tycon.

(example due to Adam Gundry)

=== Updating Explicitly Bidirectional Pattern Synonyms ===

Consider the following

```
pattern Silly{a} <- [a] where
  Silly a = [a, a]

f1 = a [5] -- 5

f2 = [5] {a = 6} -- currently [6,6]
```

=== Fixing Polymorphic Updates ===

They were fixed by adding these two lines in `dsExpr`. This might break
record updates but will be easy to fix.

```
+ ; let req_wrap = mkWpTyApps (mkTyVarTys univ_tvs)

- , pat_wrap = idHsWrapper }
+, pat_wrap = req_wrap }
```

=== Mixed selectors error ===

Note [Mixed Record Field Updates]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Consider the following pattern synonym.

    data MyRec = MyRec { foo :: Int, qux :: String }

    pattern HisRec{f1, f2} = MyRec{foo = f1, qux=f2}

This allows updates such as the following

    updater :: MyRec -> MyRec
    updater a = a {f1 = 1 }

It would also make sense to allow the following update (which we
reject).

    updater a = a {f1 = 1, qux = "two" } ==? MyRec 1 "two"

This leads to confusing behaviour when the selectors in fact refer the
same field.

    updater a = a {f1 = 1, foo = 2} ==? ???

For this reason, we reject a mixture of pattern synonym and normal
record selectors in the same update block. Although of course we still
allow the following.

    updater a = (a {f1 = 1}) {foo = 2}

    > updater (MyRec 0 "str")
    MyRec 2 "str"