Changes between Version 13 and Version 14 of Records/OverloadedRecordFields/Plan


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Timestamp:
Jul 1, 2013 8:35:59 AM (10 months ago)
Author:
adamgundry
Comment:

tell the latest story

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  • Records/OverloadedRecordFields/Plan

    v13 v14  
    3535 
    3636 
     37=== Projections ===  
     38 
     39Record field constraints are introduced by projections. If there are two or more fields `x` in scope, then an occurrence of `x` has type `a { x :: b } => a -> b` instead of generating an ambiguity error. If there is a single field `x` in scope, then it refers to the usual monomorphic record selector (ensuring backwards compatibility). If there are any normal identifiers `x` in scope (as well as fields) then a use of `x` leads to an ambiguity error. 
     40 
     41 
    3742=== Record field constraints ===  
    3843 
     
    4651Recall 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])`. 
    4752 
    48 Instances for the `Has` typeclass are implicitly generated,  corresponding to fields in datatype definitions. For example, the data type 
     53Instances for the `Has` typeclass are automatically generated (for modules with `-XOverloadedRecordFields` enabled) using the record fields that are in scope. For example, the data type 
    4954 
    5055{{{ 
     
    6267 
    6368 
    64 === Projections: the dot operator ===  
    65  
    66 Record field constraints are introduced by projections. If there are two or more fields `x` in scope, then an occurrence of `x` has type `a { x :: b } => a -> b` instead of generating an ambiguity error. If there is a single field `x` in scope, then it refers to the usual monomorphic record selector (ensuring backwards compatibility). If there are any normal identifiers `x` in scope (as well as fields) then a use of `x` leads to an ambiguity error. 
    67  
    68  
    6969=== Representation hiding === 
    7070 
    71 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. 
    72  
    73 This enables representation hiding: exporting the field selector is a proxy for permitting access to the field. For example, consider the following module: 
     71At 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 available for a module if `-XOverloadedRecordFields` is enabled for that module and the record field selector function is in scope. 
     72 
     73This enables representation hiding: just like at present, exporting the field selector permits access to the field. For example, consider the following module: 
    7474 
    7575{{{ 
     
    8080}}} 
    8181 
    82 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. 
     82Any 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 available but the instance `Has S "x" Bool` will not be. Thus `R { x :: Int }` will be solved but `S { x :: Bool }` will not. 
     83 
     84 
     85=== Multiple modules and automatic instance generation === 
     86 
     87Note that `Has` instances are generated on a per-module basis, using the fields that are in scope for that module, and automatically generated instances are never exported. Thus it doesn't matter whether `-XOverloadedRecordFields` was on in the module that defined the datatype. The availability of the instances in a particular module depends only on whether the flag is enabled for that module. 
     88 
     89Suppose module `M` imports module `N`, `N` imports module `O`, and only `N` has the extension enabled. Now `N` can project any field in scope (including those defined in `O`), but `M` cannot access any `Has` instances.  
     90 
     91This means that 
     92 * the extension is required whenever a `Has` constraint must be solved; 
     93 * no new mechanism for hiding instances is required; and 
     94 * records defined in existing modules (or other packages) without the extension can still be overloaded. 
     95 
     96 
     97=== Higher-rank fields === 
     98 
     99If a field has a rank-1 type, the `Has` encoding works fine: for example, 
     100 
     101{{{ 
     102data T = MkT { x :: forall a . a -> a } 
     103}}} 
     104 
     105gives rise to the instance 
     106 
     107{{{ 
     108instance (b ~ a -> a) => Has T "x" b 
     109}}} 
     110 
     111However, if a field has a rank-2 type or higher (so the selector function has rank at least 3), things are looking dangerously impredicative: 
     112 
     113{{{ 
     114data T b = MkT { x :: (forall a . a -> a) -> b } 
     115}}} 
     116 
     117would give 
     118 
     119{{{ 
     120instance (c ~ ((forall a . a -> a) -> b)) => Has (T b) "x" c 
     121}}} 
     122 
     123but 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 `x e`, but overloaded record fields prevent this. Non-overloaded field names are likely to be needed in this case.  
    83124 
    84125 
     
    128169 
    129170 
    130 === Virtual record fields === 
    131  
    132 It is easy to support virtual record fields, by permitting explicit `Has` instances: 
     171=== Polymorphic record update for lenses === 
     172 
     173As noted above, supporting a polymorphic version of the existing record update syntax (in its full generality) is difficult. However, even if the existing record update syntax remains monomorphic, an additional motivation for polymorphic update comes from [http://hackage.haskell.org/package/lens lens]. If we automatically generate instances of an extra class like 
     174 
     175{{{ 
     176class Set (r :: *) (x :: Symbol) (t :: *) where 
     177  setFld :: r -> t -> r 
     178}}} 
     179 
     180and supply the instance (where `&x` is used for explicit type application of the `x` argument) 
     181 
     182{{{ 
     183instance (Functor f, Has s x a, Set s x a, fs ~ f s, fa ~ f a) => Has (a -> fa) x (s -> fs) where 
     184  getFld f r = setFld &s &x &a r <$> f (getFld &s &x &a r)  
     185}}} 
     186 
     187then every record field (for which a `Set` instance can be generated) is automagically a lens. This reduces the need for the current name-mangling Template Haskell implemented in the lens library. (Note that this instance requires explicit type application, or a proxy-based workaround, in order to supply the `x` argument.) 
     188 
     189More work is needed to identify the right way to formulate the `Set` class: type-changing update requires a slightly more general version, and there is a story for [https://github.com/ekmett/lens/issues/197 multiple update]. Higher-rank fields remain problematic. 
     190 
     191 
     192=== User-defined `Has` instances === 
     193 
     194Should the user be allowed to write explicit `Has` instances? For example: 
    133195 
    134196{{{ 
     
    137199}}} 
    138200 
    139 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. 
    140  
    141 `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: 
    142  
    143 {{{ 
    144 instance Has T l () where 
    145   getFld _ = () 
    146 }}} 
    147  
    148  
    149 === Record selectors ===  
    150  
    151 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. 
    152  
    153 Optionally, we could [wiki:Records/DeclaredOverloadedRecordFields/NoMonoRecordFields 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 [wiki:Records/OverloadedRecordFields/Plan#Representationhiding representation hiding] section). 
    154  
    155 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 
    156  
    157 {{{ 
    158 x :: Has r "x" t => r -> t 
    159 x = getFld 
    160 }}} 
    161  
    162 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. 
    163  
    164  
    165 === Monomorphism restriction and defaulting === 
    166  
    167 The monomorphism restriction may cause annoyance, since 
    168  
    169 {{{ 
    170 foo = \ e -> x e 
    171 }}} 
    172  
    173 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! 
    174  
    175 For example, suppose the following definitions are in scope: 
    176 {{{ 
    177 data T = MkT { x :: Int, y :: Int } 
    178 data S = MkS { y :: Bool } 
    179 }}} 
    180  
    181 Inferring the type of `foo = \ e -> x e` 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. 
    182  
    183  
    184 === Higher-rank fields === 
    185  
    186 If a field has a rank-1 type, the `Has` encoding works fine: for example, 
    187  
    188 {{{ 
    189 data T = MkT { x :: forall a . a -> a } 
    190 }}} 
    191  
    192 gives rise to the instance 
    193  
    194 {{{ 
    195 instance (b ~ a -> a) => Has T "x" b 
    196 }}} 
    197  
    198 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: 
    199  
    200 {{{ 
    201 data T b = MkT { x :: (forall a . a -> a) -> b } 
    202 }}} 
    203  
    204 would give 
    205  
    206 {{{ 
    207 instance (c ~ ((forall a . a -> a) -> b)) => Has (T b) "x" c 
    208 }}} 
    209  
    210 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 `x e`, but overloaded record fields prevent this. Traditional monomorphic selector functions are likely to be needed in this case. 
    211  
    212  
    213 === Multiple modules and implicit instance generation === 
    214  
    215 When should `Has` instances be implicitly generated? I can think of three options: 
    216  1. If the extension is on for a module, generate instances for all datatypes in that module when checking their declarations. This means that record projections are not available to code that imports a datatype definition from a module without the flag. Some mechanism will need to restrict the scope of instances based on import/export of selectors. 
    217  2. If the extension is on for a module, generate instances for all record selectors that are in scope, but do not export them. Thus it doesn't matter whether the flag was on in the module that defined the datatype, and the availability of the instances in a particular module depends only on whether the flag is enabled. 
    218  3. If the extension is on for a module, generate and export instances for all record selectors that are in scope and do not already have instances. This is a hybrid of (1) and (2), and also requires a mechanism to restrict instance scope based on import/export of selectors. 
    219  
    220 Note that (2) can be equivalently implemented (as far as the user is concerned) by not really generating instances at all, but solving `Has` constraints directly based on the selectors in scope, much as `SingI` constraints are solved on-the-fly. 
    221  
    222 Suppose module `M` imports module `N`, `N` imports module `O`, and only `N` has the extension enabled. Under (1), `N` can project fields it defines (but not those defined in `O`), and `M` also has access to the `Has` instances for `N` (but not the dot syntax). Under (2), `N` can project any field in scope (including those defined in `O`), but `M` cannot access any `Has` instances. Under (3), `N` can project any field in scope, and `M` has access to the `Has` instances for `N` and `O` (but not fields defined in `M`). 
    223  
    224 I think (2) is probably the right choice here, because  
    225  * the extension is required whenever dot notation is used or a `Has` constraint must be solved; 
    226  * no new mechanism for hiding instances is required; and 
    227  * records defined in existing modules (or other packages) without the extension can still be used with dot notation.  
     201Even with an explicit `Has` instance as above, the name `x` will not be in scope unless a datatype has a field with name `x`. Thus it is not really useful. The previous proposal, where `(.x)` always meant "project out the `x` field", used explicit `Has` instances for virtual fields.  
     202 
     203 
     204=== Hiding record selectors ===  
     205 
     206Optionally, we could [wiki:Records/DeclaredOverloadedRecordFields/NoMonoRecordFields add a flag `-XNoMonoRecordFields`] to suppress the record selectors. Just as `-XOverloadedRecordFields` applies to a client module, and generates `Has` instances for that module, so `-XNoMonoRecordFields` in a client module would hide all the record selectors that should otherwise be in scope. The idea is that another record system could use Template Haskell to generate functions in place of selectors, and these would not clash. 
     207 
     208Since the selectors are hidden by clients (on import) rather than on export, fields can still be mentioned in import and export lists, to control access to them (as discussed in the [wiki:Records/OverloadedRecordFields/Plan#Representationhiding representation hiding] section), and used for record update. 
    228209 
    229210 
     
    248229 
    249230  baz = foo S 
     231 
     232  quux = y 
    250233}}} 
    251234 
     
    255238 
    256239When checking `baz`, the constraint `S { x :: gamma }` is generated and rejected, since the `x` from `S` is not in scope. 
     240 
     241When checking `quux`, the only `y` field in scope is of type `S -> Bool` so that is its type.