Firstly, thank you to SPJ for putting some detailed design into 'Overloaded record fields' [SPJ 1]. What I'm showing here draws heavily on the techniques he demonstrates. I wasn't happy with several parts of the design proposal; especially not with the amount of not-yet-available type machinery it involved (explicit type application, anonymous types, the kind system, String types). But then SPJ wasn't happy himself with the limitations on Record update for polymorphic fields: "This problem seems to be a killer: if record-update syntax is interpreted as a call to `set', ...". It's (too) easy to chuck rocks - the "glaring weakness" of (Haskell's record system) is still a swamp despite a pile of rocks (to mix my metaphors). This posting (as a .lhs) is the result of exploring within the "narrow issue: namespacing for record field names". I've used GHC + recent type extensions (v7.2.1). Findings in short: - I've followed SPJ with a `Has' class, and methods `get' and `set'. - I seem to have a way to update Higher-ranked fields, - and change the type of the record, - and even update Existentially-quantified fields (to a limited extent) - which should please the object-oriented oriented. I'd appreciate some feedback: - Have I misunderstood the results? - Are there still unacceptable limitations? (Certainly the error messages are impenetrable when you go wrong.) - The `Has' class, instances and type functions are ugly - can we be more elegant? (I expect the instances to be generated systematically from the data decl. So usually the application programmer won't have to see them. I like SPJ's 'Syntactic sugar for Has' to pretty-up Has constraints.) Disclaimer: - I'm keeping close to SPJ's objective of improving the situation w.r.t. name clashes for record fields. [SPJ 2] - I'm _not_ claiming this is a design for 'first class record types' /extensible records/record polymorphism. - I'm _not_ making a proposal for 'Records in Haskell'. (Not yet: if this is approach is deemed 'workable', then I have a design in mind.) - I'm _not_ envisaging anything like 'first-class labels'. (I think that idea is probably not the right objective, but that's a debate for another day.) The basic idea: - Field selection uses dot notation as reverse-application, applying a field selector via Has/get and resolving the instance to the record type and field, that is: r.fld ==> fld r, ==> get (undefined :: Proxy_fld) (r@DCons{fld}) = fld (I've used (.$) = flip ($) to fake the syntax for dot notation.) - Record construction and pattern matching with explicit data constructors works as with -XDisambiguateRecordFields - Record update uses H98 syntax, to be compiled to Has/set and resolving the instance to the record type and field with explicit data constructor: r {fld = val} ==> set fld val (r@DCons{..}) ==> DCons{fld = val, ..} -- using Puns and WildCards (I've used (.=) = set to fake the syntax for update: r.$(fld.=val).) - Of course, I can't use the field name itself, because that would clash with the H98 selector. Instead: _fld = undefined :: Proxy_fld -- for the update syntax fld_ = get (undefined :: Proxy_fld) -- for the (overloadable) -- selector function The approach uses a 'loose coupling' between the type arguments of Has/get/set, with type functions to control the linkage. (Some could be Associated Types -- a matter of taste?) The recipe needs:
{-# OPTIONS_GHC -XDisambiguateRecordFields -XNamedFieldPuns -XRecordWildCards #-} {-# OPTIONS_GHC -XTypeFamilies #-} {-# OPTIONS_GHC -XRankNTypes -XImpredicativeTypes -XGADTs -XEmptyDataDecls #-} {-# OPTIONS_GHC -XMultiParamTypeClasses -XFlexibleInstances -XUndecidableInstances #-}
module HasGetSet where
SPJ's example of a higher-ranked data type -- imported so that we have clashing declarations of `rev'
import HRrev {- data HR = HR {rev :: forall a.[a] -> [a]} -}
data Tab a b where -- a different data type with a HR field `rev' Ta :: {tag :: String, rev :: forall a_.([a_] -> [a_]), flda :: a } -> Tab a b Tb :: (Num n, Show b) => {tag :: String, fldn :: n, fldnb :: n -> b} -> Tab a b -- Existential fields (GADT syntax)
overloadable definitions for field `rev':
data Proxy_rev -- phantom, same role as SPJ's String kind "rev" _rev = undefined :: Proxy_rev rev_ r = get (undefined :: Proxy_rev) r
build some syntax to fake the dot notation, and assignment within record update
(.$) = flip ($) infixl 9 .$
-- (fld .= val) r = set fld val r -- sadly, this doesn't quite work infix 9 .= -- so define (.=) as a method in Has
test rig for higher-rank rev, per SPJ's example, and demos
testrev r = (r.$rev_ $ [True, False, False], r.$rev_ $ "hello") -- dot notation would bind tighter than func apply
rHR0 = HR{} -- `rev' is undefined, to show we can update it rHR1 = rHR0.$(_rev.=reverse) -- equivalent to rHR0{rev = reverse} -- testrev rHR1 ==> ([False,False,True],"olleh") rHR2 = rHR1.$(_rev.=(drop 1 . reverse)) -- testrev rHR2 ==> ([False,True],"lleh")
rTab0 = Ta{tag="tagged"} -- `rev' is undefined, to show we can update it rTab1 = rTab0.$(_rev.=reverse) -- testrev rTab1 ==> ([False,False,True],"olleh") rTab2 = rTab1.$(_flda.='a').$(_rev.=(reverse . take 3)) -- rTab1 :: Tab a b ; rTab2 :: Tab Char b -- testrev rTab2 ==> ([False,False,True],"leh")
here's the mechanism -- declarations for Has/get/set, and type families
class Has r fld t where get :: fld -> r -> GetTy r fld t set, (.=) :: fld -> (forall a_.SetTy r fld t a_) -> r -> SetrTy r fld t -- (fld .= val) r = set fld val r -- } sadly, neither of these work -- (.=) = set -- } (trying to define a default)
type family GetTy r fld t :: * -- the type to get at `fld' in `r' type family SetTy r fld t a_ :: * -- type to set at `fld' in updated `r' type family SetrTy r fld t :: * -- the type to set for updated `r'
The instance for field `rev' to be a Higher-ranked type.
instance (t ~ ([a_]->[a_])) => Has HR Proxy_rev t where -- the constraint needs -XUndecidableInstances get _ HR{rev} = rev set _ fn HR{..} = HR{rev = fn, ..} (_ .= fn) HR{..} = HR{rev = fn, ..}
type instance GetTy r Proxy_rev t = t -- that is ([a_] -> [a_]) from the instance constraint type instance SetTy r Proxy_rev t a_ = ([a_] -> [a_]) -- that is ([a_] -> [a_]) from `set's forall a_ type instance SetrTy r Proxy_rev t = r -- updating `rev' doesn't change the type
the above type instances apply for any record with field `rev', so also type `Tab'
instance (t ~ ([a_]->[a_])) => Has (Tab a b) Proxy_rev t where -- the constraint needs -XUndecidableInstances get _ Ta{rev} = rev set _ fn Ta{..} = Ta{rev = fn, ..} (_ .= fn) Ta{..} = Ta{rev = fn, ..}
data Proxy_flda -- `flda's type is a parameter to the data type _flda = undefined :: Proxy_flda flda_ r = get (undefined :: Proxy_flda) r instance Has (Tab a b) Proxy_flda t where -- note no constraint on `t', -- might be different to `a' for update get _ Ta{flda} = flda set _ x Ta{..} = Ta{flda = x, ..} (_ .= x) Ta{..} = Ta{flda = x, ..}
type instance GetTy (Tab a b) Proxy_flda t = a -- this is where we constrain `t' for the get type instance SetTy (Tab a b) Proxy_flda t a_ = t -- type to set is whatever we're given type instance SetrTy (Tab a b) Proxy_flda t = Tab t b -- set the result type: substitute `t' for `a'
and Has/get/set for the other fields of Tab, including the Existential
data Proxy_tag _tag = undefined :: Proxy_tag tag_ r = get (undefined :: Proxy_tag) r instance (t ~ String) => Has (Tab a b) Proxy_tag t where -- constraint on `t', because tag is always a String get _ Ta{tag} = tag get _ Tb{tag} = tag set _ x Ta{..} = Ta{tag = x, ..} set _ x Tb{..} = Tb{tag = x, ..} (_ .= x) Ta{..} = Ta{tag = x, ..} (_ .= x) Tb{..} = Tb{tag = x, ..} type instance GetTy (Tab a b) Proxy_tag t = String type instance SetTy (Tab a b) Proxy_tag t a_ = String type instance SetrTy (Tab a b) Proxy_tag t = Tab a b -- changing the tag doesn't change the record's type
data Proxy_fldn -- } the Existential fields must be set together data Proxy_fldnb -- } possible approach for multiple update _fldn = undefined :: Proxy_fldn _fldnb = undefined :: Proxy_fldnb -- no point in a 'getter' function: the types would escape
instance (t ~ (tn, tn -> b'), Show b', Num tn) -- needs -XUndecidableInstances => Has (Tab a b) (Proxy_fldn, Proxy_fldnb) t where -- the use case is: r{fldn = n, fldnb = nb} -- get _ Tb{fldn, fldnb} = (fldn, fldnb) -- No! types would escape set _ (n, nb) Tb{..} = Tb{fldn = n, fldnb = nb, ..} (_ .= (n, nb)) Tb{..} = Tb{fldn = n, fldnb = nb, ..} type instance GetTy (Tab a b) (Proxy_fldn, Proxy_fldnb) t = t -- that is: (tn, tn -> b') from the instance constraint type instance SetTy (Tab a b) (Proxy_fldn, Proxy_fldnb) t a_ = t type instance SetrTy (Tab a b) (Proxy_fldn, Proxy_fldnb) (tn, tn -> b') = Tab a b' -- changing the fields changes the type
demo
nb' Tb{fldn, fldnb} = fldnb fldn -- test rig
rTab3 = Tb{tag="tagged", fldn = 6 :: Double, fldnb = negate} -- nb' rTab3 ==> -6.0 :: Double rTab4 = rTab3.$((_fldn, _fldnb).=(5::Int, (6 +))) -- rTab3{fldn = 5, fldnb = (6 +)} -- nb' rTab4 ==> 11 :: Int
Notes/difficulties: - Language options needs -XUndecidableInstances because the approach "uses a functional-dependency-like mechanism (but using equalities)" [SPJ 1] to 'improve' type inference for some instances. Since this _doesn't_ use FunDeps, can it safely exceed Paterson Conditions? - The inability to declare (.=) = set is curious/irritating. (Neither as a free-standing function, nor a method of Has with a default implementation. And even declaring (.=) with a type signature identical to `set'. From the error messages, GHC can't match the forall'd `a_'.) If you've managed to read this far: thank you! Refs: [SPJ 1] http://hackage.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields [SPJ 2] http://hackage.haskell.org/trac/ghc/wiki/Records