#3632: lift restrictions on records with existential fields, especially in the
presence of class constraints
------------------------------------------------+---------------------------
Reporter: eflister | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 6.10.4 | Severity: normal
Keywords: existential records accessor update | Testcase:
Os: Unknown/Multiple | Architecture:
Unknown/Multiple
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the attached file demos the use of a record with an existential field with
a class constraint. it shows several cases where lifting the current
restrictions on accessing and updating this field would be both well-
defined and useful.
here is the record definition; the dur field is existential, but
constrained to be in class NoteDur.
{{{
data Note = forall x . NoteDur x => Note {
midiNum :: Int -- 0-255
, vel :: Int -- 0-255
, chan :: Int -- 0-15
, measure :: Integral a => a
, beat :: Int
, subdiv :: RealFrac a => a -- % of beat
, dur :: x
}
}}}
here is a walk through of places in the code where the current
restrictions are unnecessary and intrusive:
lines 64-95 -- these functions wouldn't be necessary if record update
syntax were enabled for both existential and non-existential fields. i
know 6.12 introduces it for non-existentials, but i don't see why it isn't
also possible for existential fields (even without a class constraint).
lines 33-35 and 60 show how much nicer it is to use regular updater syntax
in this case.
the same is true for existential accessors when there is a class
constraint -- there's no need to restrict this situation because the
accessor can have type:
fieldName :: (SomeClass x) => Record -> x
line 142 shows a case where this would be very nice to have.
line 100 + 107 -- the foralls could be implicit (maybe offer an extention
that would allow them to be implicit)
lines 134-136 compared to 138-139 show how additional factoring could be
achieved if one were allowed to pattern match on the type of an
existential with class constraints.
lines 124-127 show how it would be nice to have existential classes
lastly, allow curried updater functions:
(rec {field = }) 5
instead of
(\x -> (rec {field = x})) 5
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3632>
GHC <http://www.haskell.org/ghc/>
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