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# Overloaded record fields: a plan for implementation

This is a plan to implement overloaded record fields, along the lines of SPJ's Simple Overloaded Record Fields proposal, as a Google Summer of Code project. (See the GSoC project details, for reference.) The page on Records gives the motivation and many options. In particular, the proposal for Declared Overloaded Record Fields is closely related but makes some different design decisions.

### Motivation

A serious limitation of the Haskell record system is the inability to overload field names in record types: for example, if the data types

data Person = Person { personId :: Int, name :: String } data Address = Address { personId :: Int, address :: String }

are declared in the same module, there is no way to determine which type an occurrence of the `personId` record selector refers to. A common workaround is to use a unique prefix for each record type, but this leads to less clear code and obfuscates relationships between fields of different records. Qualified names can be used to distinguish record selectors from different modules, but using one module per record is often impractical.

Instead, we want to be able to write postfix polymorphic record projections, so that `e.personId` resolves the ambiguity using the type of `e`. In general, this requires a new form of constraint `r { x :: t }` stating that type `r` has a field `x` of type `t`. For example, the following declaration should be accepted:

getPersonId :: r { personId :: Int } => r -> Int getPersonId e = e.personId

A constraint `R { x :: t }` is solved if `R` is a datatype that has a field `x` of type `t` in scope. An error is generated if `R` has no field called `x`, it has the wrong type, or the field is not in scope.

## Design

In the sequel, we will describe the `-XOverloadedRecordFields` extension, which permits multiple field declarations with the same label, introduces new record field constraints and a new syntax for record projection.

### Record field constraints

Record field constraints `r { x :: t }` are syntactic sugar for typeclass constraints `Has r "x" t`, where

class Has (r :: *) (x :: Symbol) (t :: *) where getFld :: r -> t

Recall that `Symbol` is the kind of type-level strings. The notation extends to conjunctions: `r {x :: tx, y :: ty}` means `(Has r "x" tx, Has r "y" ty)`. Note also that `r` and `t` might be arbitrary types, not just type variables or type constructors. For example, `T (Maybe v) { x :: [Maybe v] }` means `(Has (T (Maybe b)) "x" [Maybe v])`.

Instances for the `Has` typeclass are implicitly generated, corresponding to fields in datatype definitions. For example, the data type

data T a = MkT { x :: [a] }

has the corresponding instance

instance (b ~ [a]) => Has (T a) "x" b where getFld (MkT { x = x }) = x

The `(b ~ [a])` in the instance is important, so that we get an instance match from the first two fields only. For example, if the constraint `Has (T c) "x" d` is encountered during type inference, the instance will match and generate the constraints `(a ~ c, b ~ d, b ~ [a])`.

### Projections: the dot operator

Record field constraints are introduced by projections, which are written using the dot operator with no space following it. That is, if `e :: r` then `e.x :: r { x :: t } => t`. The right section `(.x) :: a { x :: b } => a -> b` is available but the left section `(e.)` is not (what would its type be?).

The composition operator must be written with spaces on both sides, for consistency. This will break old code, but only when the `-XOverloadedRecordFields` extension is enabled. There is no ambiguity, and dot notation is already space-aware: `M.x` is a qualified name whereas `M . x` is the composition of a data constructor `M` with a function `x`. Similarly `e.x` can mean record projection, distinct from `e . x`. Note that dot (for qualified names or record projection) binds more tightly than function application, so `f e.x` means the same as `f (e.x)`. Parentheses can be used to write `(f e).x`.

### Representation hiding

At present, a datatype in one module can declare a field, but if the selector function is not exported, then the field is hidden from clients of the module. It is important to support this. Typeclasses in general have no controls over their scope, but for implicitly generated `Has` instances, the instance is in scope iff the record field selector function is.

This enables representation hiding: exporting the field selector is a proxy for permitting access to the field. For example, consider the following module:

module M ( R(x) ) where data R = R { x :: Int } data S = S { x :: Bool }

Any module that imports `M` will have access to the `x` field from `R` but not from `S`, because the instance `Has R "x" Int` will be in scope but the instance `Has S "x" Bool` will not be. Thus `R { x :: Int }` will be solved but `S { x :: Bool }` will not.

### Record update

Supporting polymorphic record update is rather more complex than polymorphic lookup. In particular:

- the type of the record may change as a result of the update;
- multiple fields must be updated simultaneously for an update to be type correct (so iterated single update is not enough); and
- records may include higher-rank components.

These problems have already been described in some detail. In the interests of doing something, even if imperfect, we plan to support only monomorphic record update. For overloaded fields to be updated, a type signature may be required in order to specify the type being updated. For example,

e { x = t }

currently relies on the name `x` to determine the datatype of the record. If this is ambiguous, a type signature can be given either to `e` or to the whole expression. Thus either

e :: T Int { x = t }

or

e { x = t } :: T Int

will be accepted. (Really only the type constructor is needed, whereas this approach requires the whole type to be specified, but it seems simpler than inventing a whole new syntax.)

## Design choices

### Record update: avoiding redundant annotations

If `e` is a variable, whose type is given explicitly in the context, we could look it up rather than requiring it to be given again. Thus

f :: T Int -> T Int f v = v { x = 5 }

would not require an extra annotation. On the other hand, we would need an annotation on the update in

\v -> (v { x = 4 }, [v, w :: T Int])

because the type of `v` is only determined later, by constraint solving.

Annoyingly, nested updates will require some annotations. In the following example, the outer update need not be annotated (since `v` is a variable that is explicitly given a type by the context) but the inner update must be (since `v.x` is not a variable):

f :: T Int -> T Int f v = v { x = v.x { y = 6 } }

### Virtual record fields

It is easy to support virtual record fields, by permitting explicit `Has` instances:

instance ctx => Has r "x" t where getFld = blah :: r -> t

Note that if `r` is a datatype with a field `x`, the virtual field will overlap, and the usual rules about overlap checking apply. Explicit instances follow the usual instance scope rules, so a virtual record field instance is always exported and imported.

`Has` constraints are slightly more general than the syntactic sugar suggests: one could imagine building constraints of the form `Has r l t` where `l` is non-canonical, for example a variable or type family. It's hard to imagine uses for such constraints, though. One idea is giving virtual fields of all possible names to a type:

instance Has T l () where getFld _ = ()

### Record selectors

Even with `-XOverloadedRecordFields` enabled, monomorphic record selector functions will be generated by default for backwards compatibility reasons, and for use when there is no ambiguity. They will not be usable if multiple selectors with the same name are in scope.

Optionally, we could add a flag `-XNoMonoRecordFields` to disable the generation of the usual monomorphic record field selector functions. This is not essential, but would free up the namespace for other record systems (e.g. **lens**). Even if the selector functions are suppressed, we still need to be able to mention the fields in import and export lists, to control access to them (as discussed in the representation hiding section).

We could also add a flag `-XPolyRecordFields` to generate polymorphic selector functions. This implies `-XNoMonoRecordFields`. For example, if a record with field `x` is declared then the function

x :: Has r "x" t => r -> t x e = e.x

would be generated. However, these have slightly odd behaviour: if two independent imported modules declare fields with the same label, they will both generate identical polymorphic selectors, so only one of them should be brought into scope.

### Monomorphism restriction and defaulting

The monomorphism restriction may cause annoyance, since

foo = \ e -> e.x

would naturally be assigned a polymorphic type. If there is only one `x` in scope, perhaps the constraint solver should pick that one (analogously to the other defaulting rules). However, this would mean that bringing a new `x` into scope (e.g. adding an import) could break code. Of course, it is already the case that bringing new identifiers into scope can create ambiguity!

For example, suppose the following definitions are in scope:

data T = MkT { x :: Int, y :: Int } data S = MkS { y :: Bool }

Inferring the type of `foo = \ e -> e.x` results in `alpha -> beta` subject to the constraint `alpha { x :: beta }`. However, the monomorphism restriction prevents this constraint from being generalised. There is only one `x` field in scope, so defaulting specialises the type to `T -> Int`. If the `y` field was used, it would instead give rise to an ambiguity error.

### Higher-rank fields

If a field has a rank-1 type, the `Has` encoding works fine: for example,

data T = MkT { x :: forall a . a -> a }

gives rise to the instance

instance (b ~ a -> a) => Has T "x" b

However, if a field has a rank-2 type or higher (so the selector function has rank at least 3), things are looking dangerously impredicative:

data T b = MkT { x :: (forall a . a -> a) -> b }

would give

instance (c ~ ((forall a . a -> a) -> b)) => Has (T b) "x" c

but this is currently forbidden by GHC, even with `-XImpredicativeTypes` enabled. Indeed, it would not be much use if it were possible, because bidirectional type inference relies on being able to immediately infer the type of neutral terms like `e.x`, but overloaded record fields prevent this. Traditional monomorphic selector functions are likely to be needed in this case.

## Example of constraint solving

Consider the example

module M ( R(R, x), S(S, y), T(T, x) ) where data R = R { x :: Int } data S = S { x :: Bool, y :: Bool } data T = T { x :: forall a . a } module N where import M foo e = e.x bar :: Bool bar = foo T baz = foo S

When checking `foo`, `e` is a variable of unknown type `alpha`, and the projection generates the constraint `alpha { x :: beta }` where `beta` is fresh. This constraint cannot be solved immediately, so generalisation yields the type `a { x :: b } => a -> b`.

When checking `bar`, the application of `foo` gives rise to the constraint `T { x :: Bool }`, which is solved since `Bool` is an instance of `forall a . a` (the type `T` gives to `x`).

When checking `baz`, the constraint `S { x :: gamma }` is generated and rejected, since the `x` from `S` is not in scope.