Version 5 (modified by adamgundry, 20 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. Additionally, a dictionary function (for the Has instance) is generated. Thus

data T = MkT { x :: Int }


$sel_x_T :: T -> Int       -- record selector (used to be called `x`)
$df_x_T  :: Has T "x" Int  -- dictionary function (coerced selector)

The naming of cats

The AvailTC Name [Name] [OccName] 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). Every field should have a corresponding selector (in the second argument). Since we have the name of the type (the first argument), we can find the selector corresponding to a field. Similar changes are required to the IEThingWith constructor of IE, which represents a thing that can be imported or exported.

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 to two entries in the GlobalRdrEnv:

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

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

The dcFields field of DataCon stores a list of

type FieldLabel = (OccName, Name)

where the first component is the field and the second is the selector function.

When the renamer handles record bindings (rnHsRecFields1 in RnPat) it replaces the fields with selector names. (Perhaps the field should be cached as well, for pretty-printing?)

Next steps

The HsExpr.HsExpr type has an extra constructor HsOverloadedRecFld OccName. When -XOverloadedRecordFields is enabled, and the renamer encounters HsVar "x" where x refers to multiple GREs that are all record fields, it replaces it with HsOverloadedRecFld "x". There needs to be a similar constructor for non-overloaded projections, so that we can pretty-print the field name but store the selector name.

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.

Automatic Has instances are generated, provided the extension is enabled, around the same time as derived instances (from deriving clauses) are generated. Every record field GRE in scope gives rise to an instance, and the dictionary function is already available. Such instances are available when typechecking the current module (in tcg_inst_env) but not exported to other modules (via tcg_insts).