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# Type-Punning Declared Overloaded Record Fields (TPDORF)
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## Thumbnail Sketch
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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:
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- Within the same module, many record types can be declared to share the field name.
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- The field name can be exported so that records in other modules can share it.
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- Furthermore, other modules can create records using that field name, and share it.
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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:
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- Some fields are both readable and updatable;
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- Some are read-only;
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- Some are completely hidden.
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In case of 'unintended' clash (another module using the same name 'by accident'), usual H98 controls apply to protect encapsulation and representation hiding.
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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:
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- The field name overloading is implemented through usual class and instance mechanisms.
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- 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.)
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### Implementation: the `Has` class, with methods `get` and `set`, and punned Types
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Record declarations generate a `Has` instance for each record type/field combination. There is a type argument for the record and the field.
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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.)
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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.)
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, 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:
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```wiki
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class Has r fld t where
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get :: r -> fld -> t -- simplified form
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set :: fld -> t -> r -> r -- where not changing record's type
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data Customer = Cust{ customer_id :: Int, ... } -- declaration syntax same as H98
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instance (t ~ Int) => Has Customer Proxy_customer_id t where -- Has instance generated, with ~ constraint
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get Cust{ customer_id } _ = customer_id -- DisambiguateRecordFields pattern
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set _ x Cust{ .. } = Cust{ customer_id = x, .. } -- RecordWildCards and NamedFieldPuns
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myCust :: Customer -- usual record decl
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... myCust{ customer_id = 27 } -- polymorphic record update
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... (customer_id myCust) ... -- field selection is func apply, or:
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... myCust.customer_id ... -- dot notation is sugar for reverse func apply
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```
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Note that the**`Has` mechanism** uses a Proxy as the type 'peg' for a field (this is the wildcard argument to `get` and `set`):
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- There must be a Proxy_type declared for each distinct field name.
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- The Proxy must be declared once, and the Proxy is then under regular name control.
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- The field selector function also must be declared once, defined using the Proxy.
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> >
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> > 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.
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To generate the correct declarations, there is to be a new `fieldLabel` sugar:
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```wiki
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fieldLabel customer_id :: r -> Int -- new declaration, desugars to Proxy and func:
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data Proxy_customer_id -- phantom
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customer_id :: r{ customer_id :: Int } => r -> Int -- r{ ... } is sugar for Has constraint
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customer_id r = get r (undefined :: Proxy_customer_id)
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set (undefined :: Proxy_customer_id) 27 myCust -- record update desugarred from above example
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```
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- (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.)
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**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):
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```wiki
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fullName r = r.firstName ++ " " ++ map toUpper r.lastName -- example adapted from SPJ
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-- dot notation binds tighter than func apply
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fullName :: r{ firstName :: String, lastName :: String} => r -> String
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-- type inferred for fullName
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-- the Has constraints use elided syntax
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```
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**Technical capabilities** and limitations for the `Has` class:
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- Monomorphic fields can be `get` and `set`.
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- Parametric polymorphic fields can be applied in polymorphic contexts, and can be `set` including changing the type of the record.
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- Higher-ranked polymorphic fields can be applied in polymorphic contexts, but cannot be set -- for the same reasons as under SORF.
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The instances use equality constraints to 'improve' types up to polymorphic.
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- `Has` uses type family functions to manage type-changing update, which adds complexity -- see Implementer's view.
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- Multiple fields can be updated in a single expression (using familiar H98 syntax), but this desugars to nested updates, which is inefficient.
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- Pattern matching and record creation using the data constructor prefixed to { ... } work as per H98 (using `DisambiguateRecordFields` and friends).
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