wiki:Records/TypePunningDeclaredOverloadedRecordFields

Version 2 (modified by AntC, 2 years ago) (diff)

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Type-Punning Declared Overloaded Record Fields (TPDORF)

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This proposal is addressing the narrow issue of namespacing for record field names by allowing more than one record in the same module to share a field name. Furthermore, it is aiming at a more structured approach to higher-ranked type fields, so that they can be updated using the same surface syntax as for other fields. This actually means a less complex implementation (compared to DORF or SORF). Specifically each field name is overloaded, and there is a Type with the same name (upshifted) so that:

  • Within the same module, many record types can be declared to share the field name.
  • The field name can be exported so that records in other modules can share it.
  • Furthermore, other modules can create records using that field name, and share it.

The export/import of both the field name and its punned Type is under usual H98 namespace and module/qualification control, so that for the record type in an importing module:

  • Some fields are both readable and updatable;
  • Some are read-only;
  • Some are completely hidden.

In case of 'unintended' clash (another module using the same name 'by accident'), usual H98 controls apply to protect encapsulation and representation hiding.

This proposal introduces several new elements of syntax, all of which desugar to use well-established extensions of ghc. The approach is yet to be prototyped, but I expect that to be possible in ghc v 7.2.1. In particular:

  • The field name overloading is implemented through usual class and instance mechanisms.
  • Field selectors are ordinary functions named for the field (but overloaded rather than H98's monomorphic), so field selection is regular function application. (There is no need for syntactically-based disambiguation at point of use.)

Implementation: the Has class, with methods get and set, and punned Types

Record declarations generate a Has instance for each record type/field combination. There is a type argument for the record and the field.

Note that SORF introduces a third argument for the field's resulting type. (This is specifically to support higher-rank typed fields; but despite the complexity it introduces, SORF has no mechanism to update h-r fields.)

TPDORF approaches h-r fields in a different way, which supports both setting and getting those fields. (I'm not claiming this is a solution, more a well-principled work-round. And not a hack.)

The main insight is that to manage large-scale data models (in which namespacing becomes onerous

, there is a third argument for the field's resulting type. This is set at the instance level using equality constraints in a functional-dependencies style. Here is the Has class (r is the record, fld is the proxy type for the field, t is the field's type), with an example record declaration, its Has instance, and examples of use:

    class Has r fld t                                             where
        get :: r -> fld -> t                                            -- simplified form
        set :: fld -> t -> r -> r                                       -- where not changing record's type

    data Customer = Cust{ customer_id :: Int, ... }                     -- declaration syntax same as H98

    instance (t ~ Int) => Has Customer Proxy_customer_id t        where -- Has instance generated, with ~ constraint
        get Cust{ customer_id } _ = customer_id                         -- DisambiguateRecordFields pattern
        set _ x Cust{ .. }        = Cust{ customer_id = x, .. }         -- RecordWildCards and NamedFieldPuns

    myCust :: Customer                                                  -- usual record decl
    ... myCust{ customer_id = 27 }                                      -- polymorphic record update
    ... (customer_id myCust) ...                                        -- field selection is func apply, or:
    ... myCust.customer_id ...                                          -- dot notation is sugar for reverse func apply

Note that the Has mechanism uses a Proxy as the type 'peg' for a field (this is the wildcard argument to get and set):

  • There must be a Proxy_type declared for each distinct field name.
  • The Proxy must be declared once, and the Proxy is then under regular name control.
  • The field selector function also must be declared once, defined using the Proxy.

It is an error to declare a record field without there being a Proxy in scope. The desugar for the data decl would create the instance to use the Proxy, but then the instance would fail.

To generate the correct declarations, there is to be a new fieldLabel sugar:

    fieldLabel customer_id :: r -> Int                                  -- new declaration, desugars to Proxy and func:
    data Proxy_customer_id                                              -- phantom
    customer_id :: r{ customer_id :: Int } => r -> Int                  -- r{ ... } is sugar for Has constraint
    customer_id r = get r (undefined :: Proxy_customer_id)

    set (undefined :: Proxy_customer_id) 27 myCust                      -- record update desugarred from above example
  • (Admittedly, this could get onerous to declare a fieldLabel for every field, even the ones that appear in a single record type. See "Option Three: Mixed In-situ and DeclaredORF: " further down this page for a suggestion of using the DORF mechanism to generate one-off H98-style fields.)

Virtual or pseudo- fields are easy to create and use, because field selection is merely function application. Virtual fields look like ordinary fields (but can't be updated, because there is no Has instance):

    fullName r = r.firstName ++ " " ++ map toUpper r.lastName           -- example adapted from SPJ
                                                                        -- dot notation binds tighter than func apply
    fullName :: r{ firstName :: String, lastName :: String} => r -> String
                                                                        -- type inferred for fullName
                                                                        -- the Has constraints use elided syntax

Technical capabilities and limitations for the Has class:

  • Monomorphic fields can be get and set.
  • Parametric polymorphic fields can be applied in polymorphic contexts, and can be set including changing the type of the record.
  • Higher-ranked polymorphic fields can be applied in polymorphic contexts, but cannot be set -- for the same reasons as under SORF.
    The instances use equality constraints to 'improve' types up to polymorphic.
  • Has uses type family functions to manage type-changing update, which adds complexity -- see Implementer's view.
  • Multiple fields can be updated in a single expression (using familiar H98 syntax), but this desugars to nested updates, which is inefficient.
  • Pattern matching and record creation using the data constructor prefixed to { ... } work as per H98 (using DisambiguateRecordFields and friends).

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