wiki:Records/DeclaredOverloadedRecordFields

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

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Declared Overloaded Record Fields (DORF)

Explained in 5 wiki pages (these proposals are linked but somewhat orthogonal):

  • No Mono Record Fields (precursor to DORF)
  • DORF -- Application Programmer's view
  • DORF -- Implementor's view
  • DORF -- Comparison to SORF
  • Dot as Postfix Funcion Apply (optional syntactic sugar)

No Mono Record Fields

This proposal is a precursor to overloaded record fields.

There is to be a compiler flag -XNoMonoRecordFields. (Default value ‑XMonoRecordFields, to give H98 behaviour.)

-XNoMonoRecordFields suppresses creating the field selector function from the field name in a record-style data declaration.

Suppressing the function frees up the namespace, to be able to experiment with various record/field approaches -- including the 'cottage industry' of Template Haskell solutions.

-XNoMonoRecordFields implies -XDisambiguateRecordFields -- otherwise the only way to access record fields is positionally. It also implies ‑XNamedFieldPuns and ‑XRecordWildCards to support field access and update. (IMHO, suppressing the field selector function should always have been part of -XDisambiguateRecordFields. I'm by no means the first to make that observation.)

Note that the field name is still valid within the scope of a pattern match, or record update inside the {...} constructor syntax.

Import/Export? and Representation hiding

Since there is no field selector function created, it can't be exported or imported.

If you say:

{-# OPTIONS_GHC -XNoMonoRecordFields                      #-}
module M( T( x ) )       where
    data T = MkT { x, y :: Int }

then the existence of field y is hidden; type T and field label x are exported, but not data constructor MkT, so x is unusable.

(Without the ‑XNoMonoRecordFields flag, field selector function x would be exported.)

Declared Overloaded Record Fields (DORF)

Application Programmer's view

This proposal is addressing the "narrow issue" of namespacing for record field names. http://hackage.haskell.org/trac/ghc/wiki/Records

I'm avoiding giving implementation details here -- see:

<DORF -- Implementor's view>; and <DORF -- comparison to SORF>

I'm not saying anything about field selection via pattern matching or record construction using explicit data constructors -- those are to behave as currently (using the approach per ‑XDisambiguateRecordFields and friends).

Currently in Haskell two records in the same module can't share a field name. This is because declaring a field name within a data decl creates a monomorphic selector function; and if it's monomorphic, we can only have one. I think the wiki is characterising the problem incorrectly:

  • it's not that the field name appearing in different record decls is ambiguous between the two record types so we need some (syntactical) way of choosing between the different definitions;
  • rather, we have one field name, and we lack the syntax/semantics for sharing it between different records.

An example: let's say I have a database application with a field (meaning type) customer_id. Then it appears in records for name and address, pricing, order entry, etc. This is not a name 'clash', it's 'intended sharing'. (It really galls me to even put it that way for explanatory purposes. Really it's the same customer_id.) In data model design you'd typically go about identifying all the fields (types aka attributes) and putting them in a data dictionary. Then you'd construct your records from them. You might (possibly) put the data dictionary in a distinct module, for easy maintenance. But you'd certainly want all the customer-related records in the same module. So a data decl:

    data Customer_NameAddress = Cust_NA { customer_id :: Int, ... } 

is _not_ declaring customer_id, it's _using_ (or instancing) an already-declared field for customer_id. Similarly, if I have a family of objects, all with a reset' method, that's not umpteen methods with a 'clash' of names, it's one method with umpteen instances. (And I might create a family of record structures to describe each object, and store the reset' method within it.)

What's more, the Haskell 98 field selector (auto-created from the data decl) is half-way to what we want. It's a function:

    customer_id :: Customer_NameAddress -> Int

The DORF proposal generalises that signature: if you want to share a field across different records, its selector function needs to be overloaded to this type:

customer_id
r{ customer_id :: Int } => r -> Int

The 'r{ ... }' is syntactic sugar for the constraint meaning "record r has field customer_id at type Int".

We need a way to declare that a name is available as an overloadable field name (roughly speaking, a class/method definition), proposed syntax:

fieldLabel customer_id
r -> Int

(The r{ ... } is added by the desugarer.)

The `-> Int' means the field's domain (type) is Int -- it's just a type. We might also want to constrain the record -- for example to be sure it is savable to persistent storage:

fieldLabel unitPrice
(Save r, Num t) => r -> t

(Again the r{ ... } gets added as a further constraint.)

Now we can use the field in a record, and that in effect declares an instance for the field/record. All these definitions are in the same module:

data Customer_NameAddress = ... (as above) data Customer_Price a = Num a => Cust_Price {

customer_id
Int,
product_id
Int,
unit_Price
a, ... } data Customer_Order = Cust_Order { customer_id :: Int, ... }

Then a field selection expression like:

... (customer_id r) ... -- H98 style field application

uses familiar type instance resolution to figure out from record type `r' how to extract the customer_id.

[Possibly that expression could be:

... r.customer_id ...

See <Dot as Postfix Func Apply> for that dot notation, but note that nothing in this proposal assumes dot notation will be needed.]

From here upwards, the r{ ... } constraint is just a constraint, and gets merged with other constraints. For example, you could define a function:

fullName r = (firstName r) ++ " " ++ (lastName r) -- per SPJ

The type inferred would be:

fullName
r{ firstName, lastName :: String} => r -> String

which is eliding:

fullName
(r{ firstName :: String}, r{ lastName :: String }) => r -> String

And if you think that's very close to the type of a field selector function, you'd be right. Here's some more examples of pseudo- or 'virtual' fields, using dot notation:

customer.fullName shape.area date.dayOfWeek -- not a field: calculated from the date name.middleInitial -- extract from the name field tuple.fst -- Prelude functions list.head list.length

[Since they're just functions, they can use dot notation -- or not: personal preference.]

Modules and qualified names for records

Do these field selector functions have a special scope in some way? No! They're just functions. They can be exported/imported.

We can't stop some other developer creating an application/package with a field customer_id which is incompatible with ours. (Say a Sales Order entry application where customer_id is a String, to merge with our Accounts Receivable.) So do we have a problem if someone wants to import both?

No! This is regular business-as-usual familiar name clash, and it's what the module system is designed to handle. The field selectors are just functions, we can use them qualified:

(My.customer_id myCust) <===> myCust.My.customer_id (Their.customer_id theirCust) <===> theirCust.Their.customer_id (My.customer_id r) -- fails if r is from the 'other' module

Import/Export? and Representation hiding

[See <No Mono Record Fields>, which is implied by DORF.]

Since there is only a single (overloaded) field selector function created, we either have to export it always, or hide it always (that is, we can't control which record instances get exported).

The field selector function is separately declared vs. the records and their fields, so must be exported separately. For example:

{-# OPTIONS_GHC -XDeclaredOverloadedRecordFields #-} module M( x ) where

fieldLabel x,y
r -> Int data T = MkT { x, y :: Int }

Here only the field selector function x' is exported. The representation is abstract, the client can't construct or dismantle a record type T'; field `y' is hidden altogether.

If you say:

{-# OPTIONS_GHC -XDeclaredOverloadedRecordFields

-XNoMonoRecordFields #-}

module M( T( x ) ) where

fieldLabel x,y
r -> Int data T = MkT { x, y :: Int }

then you are exporting the x field within record type T, but _not_ the field selector x (nor the generated type 'peg' Proxy_x).

Type T and field label x' are exported, but not data constructor MkT, so x' is unusable.

The existence of field `y' is hidden altogether.

Field Update for Overloadable Record Fields

You can (continue to) use pattern matching and data constructor tagging for record update:

case r of {

Cust_Price {unit_Price, ..}

-> Cust_Price {unit_Price = unit_Price * 1.05, .. }

} -- increases Price by 5%

(This uses ‑XDisambiguateRecordFields, -XRecordWildCards and ‑XNamedFieldPuns -- all mature GHC extensions.)

The new part is polymorphic record update:

myPrice{ unit_Price = 72 :: Int }

Returns a record with same fields as myPrice, except a different unit_Price. Note that the update can change the type of a field (if the record declaration is polymorphic).

Note that upon first encountering that expression, we don't know the record types (because unit_Price is overloaded). So the types initially inferred are:

<expr>
r { unit_Price :: Int } => r
myPrice
_r{ unit_Price :: t } => _r

That is, the update might be changing the record type as well as the field type -- in case that the record type is parametric over the field type.

Behind the scenes, the update syntax with an expression prefix to the { ... } is syntactic sugar for a call to the polymorphic record update method `set':

set (undefined
Proxy_unit_Price) (72 :: Int) myPrice

[See <DORF -- Implementor's view> for what the Proxy is doing.]

Normal type inference/instance resolution will find the record type for myPrice, and therefore the correct instance to apply the update.

You can update multiple fields at the same time:

myCustNA { firstName = "Fred", lastName = "Dagg" }

[There's a poor story to tell here in implementation terms: we split into two calls to `set', one nested inside the other. It's wasteful to build the intermediate record. Worse, the two fields' types might be parametric in the record type or polymorphically related (perhaps one is a method to apply to the other), then we get a type failure on the intermediate record.]

Some discussion threads have argued that Haskell's current record update syntax is awkward. The DORF proposal is to implement field update using a polymorphic function. Once this is implemented, alternative syntax could be explored, providing it desugars to a call to `set'.


The implementation has been prototyped in GHC 7.2.1, see http://www.haskell.org/pipermail/glasgow-haskell-users/2011-December/021298.html, and SPJ's observations/possible improvements and caveats http://www.haskell.org/pipermail/glasgow-haskell-users/2012-January/021744.html

The fact that DORF has been 'faked' in existing GHC is good evidence that it's a modest change. Furthermore, we can implement H98-style records/fields using the same mechanism.

DORF is to be enabled by a compiler flag ‑XDeclaredOverloadedRecordFields, which implies flag ‑XNoMonoRecordFields, which in turn implies ‑XDisambiguateRecordFields and -XNamedFieldPuns with ‑XRecordWildCards.

Note we do _not_ assume flag ‑XDotPostfixFuncApply; dot notation is not needed by DORF, it's purely syntactic sugar to suit the taste of the programmer.

DORF is implemented through a class Has' with methods get' and set'. (Very similar in principle to SORF.) There's an instance of Has' for each record/field combination, with the instance generated from the record declaration.

Within each instance, get/set are defined in terms of the record's data constructors, using ‑XDisambiguateRecordFields and friends.

fieldLabel declaration (data dictionary)

There is to be a new declaration type, examples:

fieldLabel customer_id
r -> Int
fieldLabel unitPrice
(Save r, Num t) => r -> t
fieldLabel monoField
Rec -> String -- equiv to H98

[`fieldLabel' is rather long as reserved words go. I'm guessing that field or label would already be heavily used in existing code. Suggestions welcome!]

The fieldLabel declaration desugars to:

data Proxy_customer_id -- phantom, a type 'peg'

customer_id
r{ customer_id :: Int } => r -> Int
customer_id r = get r (undefined
Proxy_customer_id)
unit_Price
(r{ unit_Price :: t}, Save r, Num t) => r -> t

That is: the r{ ... } constraint is added by the desugarer (and will be further desugarred to a `Has' constraint):

Syntactic sugar for `Has'

DORF steals from SORF:

r{ fld :: t } <===> Has r Proxy_fld t

Using the sugar in the surface syntax (representation) allows for some freedom in the design space 'behind the scenes'.

Should `get' have a Proxy argument? I've used a phantom/proxy type (in GHC v 7.2.1) to drive type instancing for `Has'. SORF uses a String Kind (which is only partially available with GHC v 7.4.1), with implicit type application (so `get' does not have a proxy argument). I'll leave it to the implementors to determine which works best.

get' is a method of the Has' class:

get
(Has r fld t) => r -> fld -> t

Record declaration

data Customer_NameAddress = Cust_NA { customer_id :: Int, ... }

Does _not_ create a field selector function customer_id. Instead it creates a `Has' instance:

instance (t ~ Int)

=> Has Customer_NameAddress Proxy_customer_id t where

get Cust_NA{customer_id} = customer_id

Note the bare `t' with type equality constraint. This is unashamedly stolen from SORF's "functional-dependency-like mechanism (but using equalities) for the result type". So type inference binds to this instance based only on the record type and field (type 'peg'), then 'improves' the type of the result. The definition of `get' uses ‑XDisambiguateRecordFields style (with ‑XNamedFieldPuns). [It's a wart that in the record declaration, we've had to repeat the type of customer_id when the fieldLabel decl has already stipulated Int. It is legal syntax to omit the type in the record decl, but that currently has a different effect: data ... = Cust_NA { customer_id, custName :: String, ... } currently means customer_id is to be same type as custName. Opportunity for improvement! ]

Record/field update Update uses method set' from the Has' class:

set
(Has r fld t) => fld -> t -> r -> r
`set's instances are defined using explicit data constructor:

instance (t ~ String) =>

Has Customer_NameAddress Proxy_firstName t where

set _ x (Cust_NA{..}) = Cust_NA{firstName = x, ..}

The definition of `set' uses ‑XDisambiguateRecordFields style (with ‑XNamedFieldPuns and ‑XRecordWildCards to fill in the un-named fields). Haskell's existing update syntax is desugarred to a call to `set':

myCustNA{ firstName = "John" }

===> set (undefined :: Proxy_firstName) "John" myCustNA (That is, with a name or expression preceeding the { ... }. A data constructor prefix continues to use -XDisambiguateRecordFields.) It is crucial to this proposal that we can implement a polymorphic field update function (`set'). There's a number of tricky requirements considered below. In amongst the discussion of dot notation for field selection, there have been aspersions cast on Haskell's current record update syntax. http://www.haskell.org/pipermail/haskell-cafe/2012-February/099314.html If we can support update as just a function, there's a chance we can then turn to the syntactic sugar. (For example, the application programmer can develop update idioms to suit their purpose, as just overloaded functions.)

Updating multiple fields The syntax already supports updating multiple fields:

myCustNA { firstName = "Fred", lastName = "Dagg" }

The simplest implementation is to turn this into two (nested) updates, but that makes it inefficient generating then discarding the interim result. It may also block updates of linked fields that share a type parametric in the record type. The prototype for this proposal has explored updating with a type instance pairing the two fields:

instance (t ~ (String, String)) =>

Has Customer_NameAddress

(Proxy_firstName, Proxy_lastName) t where ...

but in general, this would need instances for each perm/comb of fields. Type-changing update Haskell 98's record update can change the type of the record, by changing the type of a field that is parametric in the record's type. There has been some feedback that there are legitimate use-cases for type-changing update -- for example traversing a record structure applying some type-changing transformation. This proposal does support type changing, but at cost of considerable extra complexity. So the earlier definitions of Has/get/set have been "economical with the truth". Instead:

class Has r fld t where

get
r -> fld -> GetResult? r fld t
set
(Has (SetResult? r fld t) fld t) => fld -> t -> r -> SetResult? r fld t

The type functions are to handle the possibly-changing types:

type family GetResult? r fld t
* -- result from get
type family SetResult? r fld t
* -- result from set
For monomorphic (non-changing) fields, GetResult? returns t and SetResult? returns r, so this amounts to the simpler definitions for Has/get/set given earlier. These are type families, not associated types, because in many cases, the result from get depends only on fld', and the result from set depends only on the record type r'. In a few cases, the type function must be sensitive to the combination of field type and record type. The extra Has constraint on set's result is to 'improve' `t' by gathering constraints from the type of set's resulting record type. Note that the field value's type `t' is the type to-be in the result, _not_ the type as-was in the record being updated. So the result from set has that type `inserted'. Example, based on field unit_Price:

data Customer_Price a = Num a => Cust_Price {

...,

unit_Price
a, ... } type instance GetResult? (Customer_Price a) Proxy_unit_Price t

a -- type as is

type instance SetResult? (Customer_Price _a) Proxy_unit_Price t

Customer_Price t -- set record type per arg to set

instance (Num t) =>

Has (Customer_Price a) Proxy_unitPrice t where

get Cust_Price{unit_Price} _ = unit_Price set _ x Cust_Price{..} = Cust_Price{ unit_Price = x, .. }

(The method definitions are 'uninteresting', compared to the drama to get the types right.) The extra complexity to support changing type could be somewhat reduced using a separate `Set' class with four type parameters, including both as-was and resulting record types, and equality constraints to improve them -- rather than type family SetResult?. This would mean, though, that the type sugar for Has constraints would not be adequate. Since that sugar is to be visible but the instance definitions are to be 'internal', this proposal prefers to support the sugar.

Selecting polymorphic/higher-ranked fields Note that initialising records with polymorphic fields (using record constructor syntax) is not affected. This proposal implements selecting/applying those fields in polymorphic contexts. This includes fields with class-constrained types 'sealed' within the record. To support higher-ranked fields, this proposal follows SORF's approach (with three parameters to Has) to obtain a polymorphic type:

data HR = HR {rev :: forall a_. [a_] -> [a_]} -- per SPJ

fieldLabel rev
r -> (forall a_. [a_] -> [a_]) ===> data Proxy_rev
rev
Has r Proxy_rev t => r -> t
rev r = get r (undefined
Proxy_rev) type instance GetResult? r Proxy_rev t = t -- plain t -- field's type is whatever's there (it's opaque) -- improved by the instance constraint type instance SetResult? HR Proxy_rev t = HR -- the H-R type is hidded inside HR instance (t ~ ([a_] -> [a_])) => -- same as SORF Has HR Proxy_rev t where get HR{rev} _ = rev

Updating polymorphic/higher-ranked fields The prototype for this proposal does include a method of updating Higher-ranked fields. SPJ has quickly reviewed the prototype: "Your trick with SetTy? to support update of polymorphic fields is, I belive, an (ingenious) hack that does not scale. I think it works only for fields that are quantified over one type variable with no constraints. So, I think that update of polymorphic fields remains problematic. " Note that the "(ingenious)" and unscalable "hack" appears only in compiler-generated code. Is it a requirement to be able to update polymorphic fields? Is it sufficient to be able to update other (monomorphic) fields in records that also contain poly fields?

Representation hiding/import/export See the discussion under <Application Programmer's view> and <No Mono Record Fields>. When import/exporting do we need to also export the Proxy_type? If not exported, update syntax cannot be desuggarred to use it.)

Should application programmers declare instances for `Has'/set? Nothing so far suggests they should. (And there's obvious dangers in allowing it.) But updating through virtual fields might need it. See <DORF -- comparison to SORF>#<Virtual record selectors>.


This is a brief point-by-point comparison to http://hackage.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields

I'll only dwell on where the DORF proposal differs.

The 'Base Design' is very similar: a library class Has', methods get/set, with an instance generated for each declared record/field. Field selection (either prefix per H98 or dot notation) is desugared to a call to the get' method.

The String' type parameter to Has', and Scope control

DORF uses a Proxy type for the type/kind-level 'peg' on which to hang the instances. Using a Proxy is a superficial difference, and mostly for practical reasons -- I was building a proptotype in 7.2.1 (which doesn't have user-definable or `String' Kinds).

There is, however, a significant difference on scoping/namespacing: the Proxy type (being a type) is bound by the usual module scoping, export/import and module qualification, etc. Are (`String') Kinds bound by module scope?

The module namespacing in DORF is deliberate, and for exactly the same reason as namespacing in general: a different developer in a different module might 'accidentally' create a clashing name which is unrelated (that is, a name for a field or a record or both). As far as DORF is concerned, these are different names, so need the module name qualification.

In contrast: "The [SORF] proposal ... doesn't allow label names to be scoped: if one module internally uses "field" as a label name then another module can break the abstraction by using the same string "field"." SPJ goes on to discuss a work-round which he sees as "ugly", and concludes "we need scoped instances". DORF does not need any innovations around instances or type inference.

Should `get' have a Proxy argument? "This choice does not make a major difference either way." [SPJ]. DORF likewise doesn't care. The prototype does use a Proxy argument, because it's implemented using types not Kinds, and GHC doesn't (yet) have type application.

Higher Rank Types and Type Functions DORF follows SORF in using a "functional-dependency-like mechanism (but using equalities) " to manage the type inference for Has/get/set.

Virtual record selectors Are a valuable feature, and particularly valuable because they 'look' just like record field selectors. DORF supports them as ordinary (overloaded) functions, defined in terms of field selectors (so that their types are inferred to need the records/fields). Under DORF, virtual record selectors do not need Has' instances, so don't run into the problem of what to do about the definition for set'. (Perhaps DORF has a contrary problem: what if we do want a `set' for a virtual field? -- for example to update a fullName into firstName and lastName. There's food for thought in Wadler's original paper that led to View Patterns "Views: A way for pattern matching to cohabit with data abstraction" [1987], 4. "Viewing a complex number in cartesian and polar coordinates". We may want our implementation of complex to be abstract. We provide (pseudo-) fields to select the coordinates. Then they're ever-so like methods for an (abstract) object. Also we want the (pseudo-) fields to be updatable, which means we need an instance for Has/get/set for fields that are not explicit in the record.) Selectors as functions We need to support H98-style record selectors. One of the design objectives for DORF is that field selectors continue to be functions, just as H98. (The difference is that DORF's are overloaded whereas H98's are monomorphic.) You can use dot notation with H98 field selectors without changing the H98 record definition.

Representation hiding See the discussion under <No Mono Record Fields>. SORF has an issue, because (in effect) it needs to control export of instances. "Solving this issue is important" [SPJ]. DORF doesn't have this issue, because the field selector is just a function, so the module system can control its visibility, as usual. Syntax - The dot notation DORF pretty much agrees with SORF, including in keeping it simple by not supporting part-applied dot. Note that DORF does not rely on dot notation whatsoever -- it's purely syntactic sugar for function application. See <Dot as Postfix Function Apply> Syntax - Syntactic sugar for `Has' SPJ is spot-on. DORF follows SORF.

Record updates DORF follows SORF in having a set' method within class Has'. It's definition and instances are tricky, to support changing the type [] of records/fields, and higher-ranked fields. SPJ is "not very happy with any of this" (that is, any of the SORF approach to field update). DORF has implemented record update using (admitted) hacks. DORF has tried to hide the implementation behind syntactic sugar (for the `Has' constraint), to make available some design space for improvements. (Including possibly the Alternative proposal using Quasifunctor.) To answer SPJ's question "what does e { x = True } mean if there are lots of "x" fields in scope? ": Under DORF, there is only a single "x" in scope(overloaded to appear in many record types). So

e { x = True } :: r{ x :: Bool} => r

[] changing the type of records fields at update seems to be a required feature. DORF has implemented it to be compatible with H98, but this adds considerable complexity. We avoid the question of whether it's desirable. DORF does not have the issues trying to support or avoid set' for virtual fields, because those are not defined using Has'. (Source code that attempts to update via them, such as :

myCust { fullName = "Fred Dagg" }

will get type failure:

no instance Has Customer_NameAddress Proxy_fullName String

or earlier still (if we implement DORF using proxies):

no type Proxy_fullName

Relationship to Type Directed Name Resolution [TDNR] DORF is in some ways a reversion to something closer to TDNR (especially dot notation "works with any function"). DORF, SORF and TDNR all use a `Has' constraint, generated from the record/field declaration. Unlike TDNR, DORF has no special syntax to trigger name resolution. (DORF's dot postfix notation is just syntactic sugar for function application, and name 'resolution' is type-directed only in the sense of familiar instance resolution.) But the crucial difference is that TDNR still treats multiple declarations of the same field name (in different record types) as being different names. DORF treats these as multiple instances of the _same_ name.



Currently (in Haskell 98), declaring a record field generates a field selector function, for example:

data Rec = Rec { f :: String } -- generates:

f
Rec -> String

And f is 'just a function', to be applied in prefix form.

The DORF Overloaded Record Fields proposal also generates field selectors as 'just functions' (to be overloaded for all the record types thay appear in).

As with TDNR, I propose an additional syntactic form r.f for selecting field f from record r (of type Rec). The dynamic syntax is simply reverse application r.f <===> (f r) (So for overloaded field selectors, we use usual type-directed instance resolution.)

But dot notation must be written with no spaces around the dot -- because it has strange syntactic properties that mean it isn't 'just an operator'.

The reasoning for using dot is exactly as SPJ adumbrates in TDNR ("it is critical to support dot notation") -- essentially, it's standard practice (both for selecting fields from records in data manipulation languages such as SQL, and selecting methods from objects on OO languages). Postfix application supports an object -> action pattern of thinking: focus on the object, then apply some action; or the UNIX pipes concept of the data 'flowing' left to right.

The dot syntax is to be optional (and is orthogonal to DORF -- you can use DORF purely pre-fix). (There are voiciferous calls from some on the discussion thread not to use dot syntax, and/or not to use postfix function application. I'd rather get DORF adopted, so I don't want to get into a confrontation over lexical syntax.)

Suggested compiler flag ‑XDotPostfixFuncApply.

The syntax could use some other character than dot (hash # has been suggested), but there's a danger that is already a user-defined operator in existing code (or in some unrelated GHC extension). There are good syntactic reasons for using dot (see below), besides that it is a convention familiar from other programming paradigms.

Note this proposal differs significantly from others for dot syntax, such as: http://hackage.haskell.org/trac/haskell-prime/wiki/TypeDirectedNameResolution (TDNR) http://hackage.haskell.org/trac/ghc/wiki/Records/DotOperator

Dot notation can be used for any function, not just record fields (as with TDNR, but different to SORF). This supports pseudo- or virtual fields. The declaration:

fullName r = r.firstName ++ " " ++ r.lastName -- SPJ's example

creates fullName as just a function, not a field. But we can still use dot syntax, and here are some other similar examples:

customer.fullName shape.area -- also from SPJ date.dayOfWeek -- not a field: calculated from the date name.middleInitial -- extract from the name field tuple.fst -- Prelude functions list.head list.length

That is, we can use this syntax to 'select' attributes or properties from structures. (Because these are 'virtual', there is no update function.)

Dot notation's "strange syntactic properties"

Dot Apply must bind tighter than function application. This is unlike any other operator. We want:

map toUpper customer.lastName ===> map toUpper (lastName customer)

m.lookup key -- method `lookup' from object m ===> (lookup m) key

Postfix dots can be stacked up, and bind leftwards:

shape.position.xCoord ===> (shape.position).xCoord -- not! shape.(position.xCoord) ===> (xCoord (position shape))

But to facilitate postfix style, there are occasions where we want a loose binding form. We could of course use parentheses, but the requirement is just like loose-binding prefix function application provided by Prelude ($). Suggested operator:

(.$) = flip ($)

(This is an ordinary Haskell operator, but this proposal is asking to reserve the namespace.)

Two examples adapted from SPJ's TDNR wiki, and avoiding the 'something odd' he notices:

m.lookup key

.$ snd .$ reverse

record.list .$ reverse

.$ filter isEven .$ map double .$ foldr (+) 0 -- sum .$ ( 2) -- square

Using Dot notation amongst qualified names

This must be valid (note no embedded spaces):

MyData?.customer.Their.Module.fullName.Prelude.head.Data.Char.toUpper

The syntax rule is:

A name to the left of a dot starting upper case is a module, and the dot binds most tightly. A name to the left starting lower case is postfix apply, and binds less tightly, but tighter than usual function apply. A name at the rightmost end starting upper case is valid, it must be a data constructor. You can use parentheses to override the binding. (And parens would be needed where embedding a data constructor.)

Why no embedded spaces? -- And a code-breaking change

In case anybody's worrying about parsing the long dotted block above, this is already valid Haskell 98:

Data.Char.toUpper.Prelude.head.Prelude.tail

"It graphically appeals to the notion of a function composed of several functions", according to a poster resistant to dot as postfix function apply.

(Applying the "function" to "hello" yields 'E'.) This is equivalent:

Data.Char.toUpper . Prelude.head . Prelude.tail

That is, there's already a dot operator (function composition), so the 'good syntactic reason' (per above) for using dot as postfix function apply is that we can be sure it's already reserved as Haskell syntax.

The code-breaking change is:

Function composition will only work when the dot is surrounded by spaces.

Dot with no space either side is to change to mean postfix function application (tight-binding).

Dot with space one side only is to change to be invalid syntax -- it's too confusing to assume what's meant.

Note that if 'one-sided' dot were valid as partial application of postfix notation (section style):

(.f) ===> (\r -> r.f) -- eta-expand

===> (\r -> (f r)) -- desugar the dot ===> f -- eta-reduce

So map (.f) ts would mean map f ts.

(r.) -- makes no sense as a section, as SPJ points out.

If you really, really want a section:

(.$ f) (r .$)

There has been lengthy discussion about the interaction of dot syntax and record/field selection -- see the thread starting: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098899.html

Thank you for everybody's contributions, they've significantly tuned the proposal as it went along.

Relationship to the proposal for Declared Overloaded Record Fields (DORF)

I've concluded that dot notation is controversial (contrary to SPJ's assumption when TDNR started).

So to repeat: DORF does not rely on postfix dot notation, neither does postfix dot notation rely on DORF.

They're related because DORF relies on field selectors being functions; and field selection being function application -- for which postifx dot provides familiar syntax. (That is, familiar from other programming paradigms.)