Version 24 (modified by adamgundry, 9 months ago) (diff)


Overloaded record fields: implementation notes

Here be dragons. This page describes implementation details and progress on the implementation of the overloaded record fields plan. Development of the extension is taking place on forks of the ghc and packages-base repositories (on branch 'overloaded-record-fields').

The basic idea

Typechecking a record datatype still generates record selectors, but their names have a $sel prefix and end with the name of their type. Thus

data T = MkT { x :: Int }


$sel_x_T :: T -> Int       -- record selector (used to be called `x`)
$sel_x_T (MkT x) = x

$dfHasTx :: Has T "x"      -- corresponds to the Has instance decl
$dfHasTx = Has { getField _     = $sel_x_T
               , setField _ s e = s { x = e } }

axiom TFCo:R:GetResult : GetResult T "x" = Int  -- corresponds to the GetResult type family instance

The naming of cats

The AvailTC Name [Name] [(OccName, Name)] constructor of AvailInfo represents a type and its pieces that are in scope. Record fields are now stored in a separate list (the third argument), along with their selectors. The IEThingWith name [name] [OccName] constructor of IE, which represents a thing that can be imported or exported, only stores the field labels. SLPJ Whoa! Why should we duplicate this info. My gut feel is that the selector should not appear in the second argument. AMG Does this sound better now? It's helpful if gresFromAvail need not do lookups (it is called by the desugarer).

The Parent type has an extra constructor FldParent Name OccName that stores the parent Name and the field OccName. The GlobalRdrElt (GRE) for a field stores the selector name directly, and uses the FldParent constructor to store the field. Thus a field foo of type T gives rise this entry in the GlobalRdrEnv:

foo |->  GRE $sel_foo_T (FldParent T foo) LocalDef

SLPJ moreover I think we should store the dictionary $dfHasTfoo in the GRE for foo, not the selector. That way we get both getter and setter (via the dictionary) in one go. AMG Now I'm not sure about this. We can't build the dictionary for higher-rank fields, but they have a perfectly good selector. Moreover, if we want type-changing update there will be two dictionaries (one for the getter and one for the setter).

Note that the OccName used when adding a GRE to the environment (greOccName) now depends on the parent field: for FldParent it is the field label rather than the selector name.

The dcFields field of DataCon stores a list of FieldLabel, defined thus:

type FieldLbl a = FieldLabel {
                      flOccName  :: OccName, -- ^ Label of the field
                      flSelector :: a        -- ^ Record selector function
type FieldLabel = FieldLbl Name

whereas the ifConFields field of IfaceConDecl stores a list of FieldLbl OccName.

Source expressions

The HsExpr type has extra constructors HsOverloadedRecFld OccName and HsSingleRecFld OccName id. When -XOverloadedRecordFields is enabled, and rnExpr encounters HsVar "x" where x refers to multiple GREs that are all record fields, it replaces it with HsOverloadedRecFld "x". When the typechecker sees HsOverloadedRecFld x it emits a wanted constraint Has alpha x beta and returns type alpha -> beta where alpha and beta are fresh unification variables.

When the flag is not enabled, rnExpr turns an unambiguous record field foo into HsSingleRecFld foo $sel_foo_T. The point of this constructor is so we can pretty-print the field name but store the selector name for typechecking.

Where an AST representation type (e.g. HsRecField or ConDeclField) contained an argument of type Located id for a field, it now stores a Located RdrName for the label, and some representation of the selector. The parser uses an error thunk for the selector; it is filled in by the renamer (by rnHsRecFields1 in RnPat, and rnField in RnTypes). The new definition of ConDeclField (used in types) is:

data ConDeclField name
  = ConDeclField { cd_fld_lbl  :: Located RdrName,
                   cd_fld_sel  :: name,
                   cd_fld_type :: LBangType name, 
                   cd_fld_doc  :: Maybe LHsDocString }

The new definition of HsRecField is:

data HsRecField id arg = HsRecField {
        hsRecFieldLbl :: Located RdrName,
        hsRecFieldSel :: Either id [(id, id)],
        hsRecFieldArg :: arg,
        hsRecPun      :: Bool }

The renamer (rnHsRecFields1) supplies Left sel_name for the selector if it is unambiguous, or Right xs if it is ambiguous (because it is for a record update, and there are multiple fields with the correct label in scope). In the latter case, the possibilities xs are represented as a list of (parent name, selector name) pairs. The typechecker (tcExpr) tries three ways to disambiguate the update:

  1. Perhaps only one type has all the fields that are being updated.
  1. Use the type being pushed in, if it is already a TyConApp.
  1. Use the type signature of the record expression, if it exists and is a TyConApp.

Automatic instance generation

Has instances are generated, provided the extension is enabled, in tcInstDecls1 (the same time as derived instances (from deriving clauses) are generated). Every record field GRE in scope gives rise to an instance. Such instances are available when typechecking the current module (in tcg_inst_env) but not exported to other modules (via tcg_insts). At the moment, fresh DFunIds are generated for all instances in scope for each module, even though they are exported in interface files. Perhaps this should change.

AMG I wanted to generate each DFunId for Has once, at the field's definition site, but this causes problems for the fields defined in base, as the Has class may not be available. I've reverted to generating fresh DFunIds locally to each module for which -XOverloadedRecordFields is used. Is there a better way to do this?

As well as Has instances, instances of the type family GetResult are generated, and exactly the same question about dfun names applies to their axiom names.

Unused imports

Unused imports and generation of the minimal import list (RnNames.warnUnusedImportDecls) currently show selector names rather than labels. We may need to create a mapping from dfun names to field labels (cf. kids_env in RnNames.reportUnusedNames) to know how to print them. Moreover, things are a bit trickier with -XOverloadedRecordFields enabled. Quoting SLPJ:

On unused imports, it's not just an implementation question.

module A where
  data T = MkT { x,y:Int }

module B where
  data S = MkS { x,y::Bool }

module C where
  import A( T(x) )
  import B( S(x) )

  foo :: T -> Int
  foo r = r.x + 2

Now, do we expect to report the 'x' in S(x) import as unused? Actually the entire 'import B' is unused. Only the typechecker will eventually know that. But I think the type checker does actually record which instances are used, so perhaps we can make use of that info to give accurate unused-import info.

GADT record updates

Annoyingly, the generated code for setField doesn't typecheck for GADTs, because of #2595. Consider the example

data W a where
    MkW :: a ~ b => { x :: a, y :: b } -> W (a, b)

At the moment, my code generates

-- setField :: proxy "x" -> W (a, b) -> a -> W (a, b)
setField _ s e = s { x = e }

but this record update is rejected by the typechecker, even though it is perfectly sensible. The alternative is for me to generate the explicit update

setField _ (MkW _ y) x = MkW x y

which is fine, but rather long-winded if there are many constructors or fields. Essentially this is doing the job of the desugarer for record updates. I wonder if it would be easier to fix #2595. Perhaps not; the general case makes my brain hurt. I only need a rather special case: updating one field, where either the type does not change, or none of the local constraints mention changing type variables.

Note that W does not admit type-changing single update for either field, because of the a ~ b constraint. Without it, though, type-changing update should be allowed.

Outstanding bugs

Some of the tests fail for the ghci way because the System.IO and GHC.TopHandler modules are not loaded automatically. I'm not sure if this is my fault, or if the problem existed in HEAD when I branched. For the moment, I've tweaked the testsuite to load the necessary modules.

Tests in need of attention:

  • rename/should_fail/T5892a (accept changed output)
  • ghci/scripts/ghci042 (accept changed output)
  • ghci/prog002 prog003 scripts/ghci029 ghci036 ghci037 (scope issues in GHCi)

To do

Simple update is implemented, using the design here. The next step is to implement type-changing update.

Implement the syntactic sugar r { x :: t }.

Test the interaction between fields and qualified names. In particular, a qualified name can be used for unambiguous identification of fields (e.g. in updates) but should probably not be used as an overloaded variable.

Universally quantified fields should result in a warning being emitted and no Has instance generated. What about existentially quantified fields (naughty record selectors)?

How should deprecation work for fields? Not at all?

Consider error messages: can we give helpful feedback so users don't have to understand the details of the implementation? For example, unsolved Accessor or Has constraints could be given nicer messages.

Consider defaulting Accessor p to p = (->), and defaulting Has r "f" t constraints where there is only one datatype with a field f in scope.