• Matthew Pickering's avatar
    Record pattern synonyms · 2a74a64e
    Matthew Pickering authored
    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"
    2a74a64e
TcPat.hs 51.3 KB