Re: [Haskell-cafe] What Haskell Records Need
Evan Laforge wrote: I consider that a strength of the lens approach. If I say 'set (a.b.c.d) 42 record', 'a', 'b' etc. don't have to be record fields, I can swap them out for other lenses later on. I can also easily precompose, e.g. 'setThis = a . b; setThat = b . c' and encourage people to use the composed ones (or require via export lists). This corresponds to asking in that it introduces a point of abstraction where I can change all access / modification in one place, or a module can retain control by only exporting the composed version. The same is true with SEC functions: personsSalary' :: (Salary - Salary) - Person - Person personsSalary' = job' . salary' Here I've created a new updater that is composed of 2 that are generated for me (from the examples given in the original email). I can export whichever of these functions I like, generated or otherwise, and keep as much abstraction as I like! The nice part about the SEC functions is that they compose as regular functions. Lenses are super powerful in that they form a category. Unfortunately using categories other than functions feels a tad unwieldy because you have to hide something from prelude and then import Category. (A bit like exceptions, currently). If you like the look of set with lenses, you could define a helper function to use with SEC updaters. set :: ((b - a) - c) - a - c set sec = sec . const --and then use it like so: setPersonsSalary :: Salary - Person - Person setPersonsSalary salary = set personsSalary' salary With it you can use an updater as a setter. I'd like to reiterate one of finer points of the original proposal. The compiler could disallow using old-style update syntax for fields whose SEC update function is not in scope, giving us fine-grained control over access and update. On the other hand we currently have to create new functions to achieve this (exporting the getter means exporting the ability to update [using update syntax] as well, currently). And now back to lenses: it is really convenient how lenses let you compose the getter and setter together. I don't recall too many cases where having the getter and setter and modifier all in one place was terribly useful. Could anyone give me an example? But again, where that is useful, a lens can be created from a getter and a SEC updater. Thoughts? --Jonathan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What Haskell Records Need
On Fri, Aug 3, 2012 at 10:11 AM, Jonathan Geddes
geddes.jonat...@gmail.comwrote:
The nice part about the SEC functions is that
they compose as regular functions. Lenses are
super powerful in that they form a category.
Unfortunately using categories other than
functions feels a tad unwieldy because you
have to hide something from prelude and then
import Category. (A bit like exceptions,
currently).
FWIW this is also true for van Laarhoven lenses[1]
type FTLens a b = forall f. Functor f = (b - f b) - (a - f a)
newtype Const a b = Const { unConst :: a } deriving Functor
get :: FTLens a b - a - b
get ft = unConst . ft Const
{-
ft :: forall f. (b - f b) - (a - f a)
Const :: forall x. b - Const b x
ft Const :: a - Const b a
-}
newtype Id a = Id { unId :: a } deriving Functor
set :: FTLens a b - b - a - a
set ft b = unId . ft (\_ - Id b)
modify :: FTLens a b - (b - b) - a - a
modify ft k = unId . ft (Id . k)
-- example
fstLens :: FTLens (a,b) a
fstLens aToFa (a,b) = (,b) $ aToFa a
-- and you get
compose :: FTLens b c - FTLens a b - FTLens a c
compose = (.)
identity :: FTLens a a
identity = id
If you like the look of set with lenses,
you could define a helper function to use
with SEC updaters.
set :: ((b - a) - c) - a - c
set sec = sec . const
--and then use it like so:
setPersonsSalary :: Salary - Person - Person
setPersonsSalary salary = set personsSalary' salary
With it you can use an updater as a setter.
I'd like to reiterate one of finer points of
the original proposal.
The compiler could disallow using old-style
update syntax for fields whose SEC update
function is not in scope, giving us
fine-grained control over access and update.
On the other hand we currently have to create
new functions to achieve this (exporting the
getter means exporting the ability to update
[using update syntax] as well, currently).
And now back to lenses:
it is really convenient how lenses let you compose the getter
and setter together.
I don't recall too many cases where having the
getter and setter and modifier all in one
place was terribly useful. Could anyone give
me an example? But again, where that is
useful, a lens can be created from a getter
and a SEC updater.
Thoughts?
--Jonathan
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Re: [Haskell-cafe] What Haskell Records Need
Oops, forgot my references
[1] Original post:
http://www.twanvl.nl/blog/haskell/cps-functional-references
[2] polymorphic update support: http://r6.ca/blog/20120623T104901Z.html
[3] another post about these:
http://comonad.com/reader/2012/mirrored-lenses/
On Fri, Aug 3, 2012 at 1:53 PM, Ryan Ingram ryani.s...@gmail.com wrote:
On Fri, Aug 3, 2012 at 10:11 AM, Jonathan Geddes
geddes.jonat...@gmail.com wrote:
The nice part about the SEC functions is that
they compose as regular functions. Lenses are
super powerful in that they form a category.
Unfortunately using categories other than
functions feels a tad unwieldy because you
have to hide something from prelude and then
import Category. (A bit like exceptions,
currently).
FWIW this is also true for van Laarhoven lenses[1]
type FTLens a b = forall f. Functor f = (b - f b) - (a - f a)
newtype Const a b = Const { unConst :: a } deriving Functor
get :: FTLens a b - a - b
get ft = unConst . ft Const
{-
ft :: forall f. (b - f b) - (a - f a)
Const :: forall x. b - Const b x
ft Const :: a - Const b a
-}
newtype Id a = Id { unId :: a } deriving Functor
set :: FTLens a b - b - a - a
set ft b = unId . ft (\_ - Id b)
modify :: FTLens a b - (b - b) - a - a
modify ft k = unId . ft (Id . k)
-- example
fstLens :: FTLens (a,b) a
fstLens aToFa (a,b) = (,b) $ aToFa a
-- and you get
compose :: FTLens b c - FTLens a b - FTLens a c
compose = (.)
identity :: FTLens a a
identity = id
If you like the look of set with lenses,
you could define a helper function to use
with SEC updaters.
set :: ((b - a) - c) - a - c
set sec = sec . const
--and then use it like so:
setPersonsSalary :: Salary - Person - Person
setPersonsSalary salary = set personsSalary' salary
With it you can use an updater as a setter.
I'd like to reiterate one of finer points of
the original proposal.
The compiler could disallow using old-style
update syntax for fields whose SEC update
function is not in scope, giving us
fine-grained control over access and update.
On the other hand we currently have to create
new functions to achieve this (exporting the
getter means exporting the ability to update
[using update syntax] as well, currently).
And now back to lenses:
it is really convenient how lenses let you compose the getter
and setter together.
I don't recall too many cases where having the
getter and setter and modifier all in one
place was terribly useful. Could anyone give
me an example? But again, where that is
useful, a lens can be created from a getter
and a SEC updater.
Thoughts?
--Jonathan
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Re: [Haskell-cafe] What Haskell Records Need
On 2/08/2012, at 5:34 PM, Jonathan Geddes wrote: Ouch! And that's not even very deeply nested. Imagine 4 or 5 levels deep. It really makes Haskell feel clunky next to `a.b.c.d = val` that you see in other languages. I was taught that this kind of thing violates the Law of Demeter and that an object should not be mutating the parts of an acquaintance's parts, but should ask the acquaintance to do so. I'd say that a.b.c.d = val is at the very least a sign that some encapsulation did not happen. Semantic editor combinators are ingenious, but they make me feel somewhat uneasy, in that they really are in some sense about the *syntax* (or maybe the concrete representation) of things rather than their *semantics* (or maybe I mean the abstract value). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What Haskell Records Need
Isn't this exactly the problem solved by all the lens packages?
Current popular ones are fclabels [0] and data-lens [1].
[0] http://hackage.haskell.org/package/fclabels
[1] http://hackage.haskell.org/package/data-lens
On Thu, Aug 2, 2012 at 7:34 AM, Jonathan Geddes
geddes.jonat...@gmail.com wrote:
Greetings,
tl;dr - What Haskell Records need are
semantic editor combinators for free.
I know this is yet another Record proposal
among many, but none of them out there
strike me as being exactly what I want in
Haskell.
Take the following types from a contrived
example.
type Salary = Integer
data Job = Job
{ title :: String
, salary :: Salary
}
data Person = Person
{ name :: String
, job :: Job
}
Since I've used record syntax, I get
getter/accessor functions (title, salary,
name, job) for free. Now suppose I want to
create an aggregate getter function: return
the salary of a given person. Piece of cake,
it's just function composition
getSalary :: Person - Salary
getSalary = salary . job
Done! Now suppose I want to write a
setter/mutator function for the same nested
field
setSalaryMessy :: Salary - Person - Person
setSalaryMessy newSalary person =
person {
job = (job person) {
salary = newSalary
}
}
Ouch! And that's not even very deeply nested.
Imagine 4 or 5 levels deep. It really makes
Haskell feel clunky next to `a.b.c.d = val`
that you see in other languages. Of course
immutability means that the semantics of
Haskell are quite different (we're creating
new values here, not updating old ones) but
it's still common to model change using these
kinds of updates.
What if along with the free getters that
the compiler generates when we use record
syntax, we also got semantic editor
combinator (SEC) functions[0] that could be
used as follows?
setSalary newSalary = job' $ salary' (const newSalary)
giveRaise amount = job' $ salary' (+amount)
givePercentRaise percent = job' $ salary' (*(1+percent))
For each field x, the compiler generates a
function x' (the tic is mnemonic for change).
These little functions aren't hard to write,
but they're classic boilerplate.
job' :: (Job - Job) - Person - Person
job' f person = person {job = f $ job person}
salary' :: (Salary - Salary) - Job - Job
salary' f job = job { salary = f $ salary job}
These type of utility functions are a dream
when working with any reference type or
State Monad.
modify $ givePercentRaise 0.25
The compiler could also generate polymorphic
SEC functions for polymorphic fields.
Further, the compiler could disallow using
old-style update syntax for fields whose SEC
update function is not in scope, giving us
fine-grained control over access and update.
On the other hand we currently have to create
new functions to achieve this (exporting the
getter means exporting the ability to update
as well, currently).
Of course this doesn't address the
namespacing issues with records, but it is
likely nicely orthogonal to other proposals
which do.
Also note that there's a package on hackage [1]
that will generate SEC functions using TH.
It's nice, but I prefer the style of field
names used above for updaters (field' vs
editField).
Let me know what you think. I'll write up an
official proposal if there's a bit of
general interest around this.
Thanks for reading,
--Jonathan
[0] - http://conal.net/blog/posts/semantic-editor-combinators
[1] -
http://hackage.haskell.org/packages/archive/sec/0.0.1/doc/html/Data-SemanticEditors.html
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Re: [Haskell-cafe] What Haskell Records Need
On 2 Aug 2012, at 09:25, Erik Hesselink wrote:
Isn't this exactly the problem solved by all the lens packages?
Current popular ones are fclabels [0] and data-lens [1].
[0] http://hackage.haskell.org/package/fclabels
[1] http://hackage.haskell.org/package/data-lens
Not sure what all of these do, but I have a simple solution I use
in my work:
Take the following types from a contrived
example.
type Salary = Integer
data Job = Job
{ title :: String
, salary :: Salary
}
Rather than setTitle :: String - Job - Job we lift the first argument
and define
setfTitle :: (String - String) - Job - Job
setfTitle f jobrec = jobrec{ title = f $ title jobrec }
then setTitle = setfTitle . const
This is all just boilerplate, so we continue
setfSalary :: (Salary - Salary) - Job - Job
setfSalary f jobrec = jobrec{ salary = f $ salary jobrec }
data Person = Person
{ name :: String
, job :: Job
}
setfName :: (String - String) - Person - Person
setfName f prec = prec{ name = f $ name prec }
setfJob :: (Job - Job) - Person - Person
setfJob f prec = prec{ job = f $ job prec }
Now we can use function composition to do two levels
setfTitleInPerson :: (String - String) - Person - Person
setfTitleInPerson = setfJob . setfTitle
setTitleInPerson :: String - Person - Person
setTitleInPerson = setfTitleInPerson . const
Simple function composition works to put these together...
I was frustrated by this problem a while back, and decided to approach it
formally
(I write literate Haskell/LateX documents), and went to work, doing the math
with the intention of writing a suitable combinator, until I discovered I didn't
need one lifting from X - R - R to (X - X) - R - R gave me all I
needed...
Since I've used record syntax, I get
getter/accessor functions (title, salary,
name, job) for free. Now suppose I want to
create an aggregate getter function: return
the salary of a given person. Piece of cake,
it's just function composition
getSalary :: Person - Salary
getSalary = salary . job
Done! Now suppose I want to write a
setter/mutator function for the same nested
field
setSalaryMessy :: Salary - Person - Person
setSalaryMessy newSalary person =
person {
job = (job person) {
salary = newSalary
}
}
Ouch! And that's not even very deeply nested.
Imagine 4 or 5 levels deep. It really makes
Haskell feel clunky next to `a.b.c.d = val`
that you see in other languages. Of course
immutability means that the semantics of
Haskell are quite different (we're creating
new values here, not updating old ones) but
it's still common to model change using these
kinds of updates.
What if along with the free getters that
the compiler generates when we use record
syntax, we also got semantic editor
combinator (SEC) functions[0] that could be
used as follows?
setSalary newSalary = job' $ salary' (const newSalary)
giveRaise amount = job' $ salary' (+amount)
givePercentRaise percent = job' $ salary' (*(1+percent))
For each field x, the compiler generates a
function x' (the tic is mnemonic for change).
These little functions aren't hard to write,
but they're classic boilerplate.
job' :: (Job - Job) - Person - Person
job' f person = person {job = f $ job person}
salary' :: (Salary - Salary) - Job - Job
salary' f job = job { salary = f $ salary job}
These type of utility functions are a dream
when working with any reference type or
State Monad.
modify $ givePercentRaise 0.25
The compiler could also generate polymorphic
SEC functions for polymorphic fields.
Further, the compiler could disallow using
old-style update syntax for fields whose SEC
update function is not in scope, giving us
fine-grained control over access and update.
On the other hand we currently have to create
new functions to achieve this (exporting the
getter means exporting the ability to update
as well, currently).
Of course this doesn't address the
namespacing issues with records, but it is
likely nicely orthogonal to other proposals
which do.
Also note that there's a package on hackage [1]
that will generate SEC functions using TH.
It's nice, but I prefer the style of field
names used above for updaters (field' vs
editField).
Let me know what you think. I'll write up an
official proposal if there's a bit of
general interest around this.
Thanks for reading,
--Jonathan
[0] - http://conal.net/blog/posts/semantic-editor-combinators
[1] -
http://hackage.haskell.org/packages/archive/sec/0.0.1/doc/html/Data-SemanticEditors.html
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Andrew Butterfield
Re: [Haskell-cafe] What Haskell Records Need
Ah yes - the joy of Haskell It so easy to roll your own, rather than search to find someone else's (better/more elegant) solution... :-) On 2 Aug 2012, at 11:41, Erik Hesselink wrote: On Thu, Aug 2, 2012 at 12:30 PM, Andrew Butterfield andrew.butterfi...@scss.tcd.ie wrote: On 2 Aug 2012, at 09:25, Erik Hesselink wrote: Isn't this exactly the problem solved by all the lens packages? Current popular ones are fclabels [0] and data-lens [1]. [0] http://hackage.haskell.org/package/fclabels [1] http://hackage.haskell.org/package/data-lens Not sure what all of these do, but I have a simple solution I use in my work: They do exactly that. They create 'lenses' which are getters/setters/modifiers combined, and allow you to compose these to get/set/modify deep inside nested data types. Look at the examples in the fclabels documentation [2] for more details. [2] http://hackage.haskell.org/packages/archive/fclabels/1.1.4/doc/html/Data-Label.html Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations Methods Research Group Director of Teaching and Learning - Undergraduate, School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What Haskell Records Need
On Thu, Aug 2, 2012 at 12:30 PM, Andrew Butterfield andrew.butterfi...@scss.tcd.ie wrote: On 2 Aug 2012, at 09:25, Erik Hesselink wrote: Isn't this exactly the problem solved by all the lens packages? Current popular ones are fclabels [0] and data-lens [1]. [0] http://hackage.haskell.org/package/fclabels [1] http://hackage.haskell.org/package/data-lens Not sure what all of these do, but I have a simple solution I use in my work: They do exactly that. They create 'lenses' which are getters/setters/modifiers combined, and allow you to compose these to get/set/modify deep inside nested data types. Look at the examples in the fclabels documentation [2] for more details. [2] http://hackage.haskell.org/packages/archive/fclabels/1.1.4/doc/html/Data-Label.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What Haskell Records Need
Richard O'Keefe Said: Ouch! And that's not even very deeply nested. Imagine 4 or 5 levels deep. It really makes Haskell feel clunky next to `a.b.c.d = val` that you see in other languages. I was taught that this kind of thing violates the Law of Demeter and that an object should not be mutating the parts of an acquaintance's parts, but should ask the acquaintance to do so. I'd say that a.b.c.d = val is at the very least a sign that some encapsulation did not happen. Absolutely! But in Haskell how do you do the asking? I guess that's what I'm proposing is a built in way of doing just that! I'm shooting for as-easy-as the built in getters. Erik Hesselink said: Isn't this exactly the problem solved by all the lens packages? Yes it is. I think the existence of these packages along with all the proposals to change records is an indication that something is missing from the language as a whole. What I'm proposing is that the language give you something that is lightweight and easy to use to address this issue. You can still use lenses on top of all of this. makeLens myField myField' If I remember correctly, one of the problems with lenses is that they cannot support polymorphic updates (updates which change a type variable of the data). SEC functions, on the other hand support polymorphic updates. --Jonathan On Thu, Aug 2, 2012 at 4:48 AM, Andrew Butterfield andrew.butterfi...@scss.tcd.ie wrote: Ah yes - the joy of Haskell It so easy to roll your own, rather than search to find someone else's (better/more elegant) solution... :-) On 2 Aug 2012, at 11:41, Erik Hesselink wrote: On Thu, Aug 2, 2012 at 12:30 PM, Andrew Butterfield andrew.butterfi...@scss.tcd.ie wrote: On 2 Aug 2012, at 09:25, Erik Hesselink wrote: Isn't this exactly the problem solved by all the lens packages? Current popular ones are fclabels [0] and data-lens [1]. [0] http://hackage.haskell.org/package/fclabels [1] http://hackage.haskell.org/package/data-lens Not sure what all of these do, but I have a simple solution I use in my work: They do exactly that. They create 'lenses' which are getters/setters/modifiers combined, and allow you to compose these to get/set/modify deep inside nested data types. Look at the examples in the fclabels documentation [2] for more details. [2] http://hackage.haskell.org/packages/archive/fclabels/1.1.4/doc/html/Data-Label.html Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations Methods Research Group Director of Teaching and Learning - Undergraduate, School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What Haskell Records Need
I'm new to Haskell, but I do like your idea. I prefer this as a built-in feature because it will create a standard way of doing this, making the question wich package should I use to get mutatos? lens-foo, lens-bar, monad-lens, lens-lens-foo-bar, ...? simply go away. So, yes, I up-vote your idea to write an official proposal. Thiago. 2012/8/2 Jonathan Geddes geddes.jonat...@gmail.com: Richard O'Keefe Said: Ouch! And that's not even very deeply nested. Imagine 4 or 5 levels deep. It really makes Haskell feel clunky next to `a.b.c.d = val` that you see in other languages. I was taught that this kind of thing violates the Law of Demeter and that an object should not be mutating the parts of an acquaintance's parts, but should ask the acquaintance to do so. I'd say that a.b.c.d = val is at the very least a sign that some encapsulation did not happen. Absolutely! But in Haskell how do you do the asking? I guess that's what I'm proposing is a built in way of doing just that! I'm shooting for as-easy-as the built in getters. Erik Hesselink said: Isn't this exactly the problem solved by all the lens packages? Yes it is. I think the existence of these packages along with all the proposals to change records is an indication that something is missing from the language as a whole. What I'm proposing is that the language give you something that is lightweight and easy to use to address this issue. You can still use lenses on top of all of this. makeLens myField myField' If I remember correctly, one of the problems with lenses is that they cannot support polymorphic updates (updates which change a type variable of the data). SEC functions, on the other hand support polymorphic updates. --Jonathan On Thu, Aug 2, 2012 at 4:48 AM, Andrew Butterfield andrew.butterfi...@scss.tcd.ie wrote: Ah yes - the joy of Haskell It so easy to roll your own, rather than search to find someone else's (better/more elegant) solution... :-) On 2 Aug 2012, at 11:41, Erik Hesselink wrote: On Thu, Aug 2, 2012 at 12:30 PM, Andrew Butterfield andrew.butterfi...@scss.tcd.ie wrote: On 2 Aug 2012, at 09:25, Erik Hesselink wrote: Isn't this exactly the problem solved by all the lens packages? Current popular ones are fclabels [0] and data-lens [1]. [0] http://hackage.haskell.org/package/fclabels [1] http://hackage.haskell.org/package/data-lens Not sure what all of these do, but I have a simple solution I use in my work: They do exactly that. They create 'lenses' which are getters/setters/modifiers combined, and allow you to compose these to get/set/modify deep inside nested data types. Look at the examples in the fclabels documentation [2] for more details. [2] http://hackage.haskell.org/packages/archive/fclabels/1.1.4/doc/html/Data-Label.html Andrew Butterfield Tel: +353-1-896-2517 Fax: +353-1-677-2204 Lero@TCD, Head of Foundations Methods Research Group Director of Teaching and Learning - Undergraduate, School of Computer Science and Statistics, Room G.39, O'Reilly Institute, Trinity College, University of Dublin http://www.scss.tcd.ie/Andrew.Butterfield/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What Haskell Records Need
On Thu, Aug 2, 2012 at 9:00 AM, Jonathan Geddes geddes.jonat...@gmail.com wrote: Richard O'Keefe Said: Ouch! And that's not even very deeply nested. Imagine 4 or 5 levels deep. It really makes Haskell feel clunky next to `a.b.c.d = val` that you see in other languages. I was taught that this kind of thing violates the Law of Demeter and that an object should not be mutating the parts of an acquaintance's parts, but should ask the acquaintance to do so. I'd say that a.b.c.d = val is at the very least a sign that some encapsulation did not happen. Absolutely! But in Haskell how do you do the asking? I guess that's what I'm proposing is a built in way of doing just that! I'm shooting for as-easy-as the built in getters. I consider that a strength of the lens approach. If I say 'set (a.b.c.d) 42 record', 'a', 'b' etc. don't have to be record fields, I can swap them out for other lenses later on. I can also easily precompose, e.g. 'setThis = a . b; setThat = b . c' and encourage people to use the composed ones (or require via export lists). This corresponds to asking in that it introduces a point of abstraction where I can change all access / modification in one place, or a module can retain control by only exporting the composed version. Erik Hesselink said: Isn't this exactly the problem solved by all the lens packages? Yes it is. I think the existence of these packages along with all the proposals to change records is an indication that something is missing from the language as a whole. What I'm proposing is that the language give you something that is lightweight and easy to use to address this issue. You can still use lenses on top of all of this. I put up a record suggestion a while back that was in two parts, one was a default lens implementation and the ability to write 'deriving (Lens)' on a record to create the lenses. The other was some magic syntax to make it easier to type in the lens names. Actually it was mostly magic syntax, since 'deriving (Lens)' pretty much speaks for itself, though you'd need to include a default lens implementation in the stdlib. I think that's a good idea anyway, but on the other hand people are still innovating in lens land. But back on the first hand again, threat of inclusion in the stdlib might force a much-needed consolidation and polishing in the lens world (pun honestly not intended). Automatically creating something (semantic editors) which isn't lenses but makes it easier to write lenses is an interesting compromise, though it is really convenient how lenses let you compose the getter and setter together. But I've found that even one line of boilerplate for each record field is already enough to discourage me from writing one for each record field since it only pays off if there's a nested update. If I remember correctly, one of the problems with lenses is that they cannot support polymorphic updates (updates which change a type variable of the data). SEC functions, on the other hand support polymorphic updates. This has been solved, yes? I haven't gotten time to investigate fully, but: http://hackage.haskell.org/package/lens-family I should look into it and update the lensy-record proposal if they're appropriate. Though it would be nice to see feedback on it, point out a fatal flaw, or at least someone could mention they read it, so I can know whether or not it's worth spending the time: http://hackage.haskell.org/trac/ghc/wiki/Records/SyntaxDirectedNameResolution ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] What Haskell Records Need
Greetings,
tl;dr - What Haskell Records need are
semantic editor combinators for free.
I know this is yet another Record proposal
among many, but none of them out there
strike me as being exactly what I want in
Haskell.
Take the following types from a contrived
example.
type Salary = Integer
data Job = Job
{ title :: String
, salary :: Salary
}
data Person = Person
{ name :: String
, job :: Job
}
Since I've used record syntax, I get
getter/accessor functions (title, salary,
name, job) for free. Now suppose I want to
create an aggregate getter function: return
the salary of a given person. Piece of cake,
it's just function composition
getSalary :: Person - Salary
getSalary = salary . job
Done! Now suppose I want to write a
setter/mutator function for the same nested
field
setSalaryMessy :: Salary - Person - Person
setSalaryMessy newSalary person =
person {
job = (job person) {
salary = newSalary
}
}
Ouch! And that's not even very deeply nested.
Imagine 4 or 5 levels deep. It really makes
Haskell feel clunky next to `a.b.c.d = val`
that you see in other languages. Of course
immutability means that the semantics of
Haskell are quite different (we're creating
new values here, not updating old ones) but
it's still common to model change using these
kinds of updates.
What if along with the free getters that
the compiler generates when we use record
syntax, we also got semantic editor
combinator (SEC) functions[0] that could be
used as follows?
setSalary newSalary = job' $ salary' (const newSalary)
giveRaise amount = job' $ salary' (+amount)
givePercentRaise percent = job' $ salary' (*(1+percent))
For each field x, the compiler generates a
function x' (the tic is mnemonic for change).
These little functions aren't hard to write,
but they're classic boilerplate.
job' :: (Job - Job) - Person - Person
job' f person = person {job = f $ job person}
salary' :: (Salary - Salary) - Job - Job
salary' f job = job { salary = f $ salary job}
These type of utility functions are a dream
when working with any reference type or
State Monad.
modify $ givePercentRaise 0.25
The compiler could also generate polymorphic
SEC functions for polymorphic fields.
Further, the compiler could disallow using
old-style update syntax for fields whose SEC
update function is not in scope, giving us
fine-grained control over access and update.
On the other hand we currently have to create
new functions to achieve this (exporting the
getter means exporting the ability to update
as well, currently).
Of course this doesn't address the
namespacing issues with records, but it is
likely nicely orthogonal to other proposals
which do.
Also note that there's a package on hackage [1]
that will generate SEC functions using TH.
It's nice, but I prefer the style of field
names used above for updaters (field' vs
editField).
Let me know what you think. I'll write up an
official proposal if there's a bit of
general interest around this.
Thanks for reading,
--Jonathan
[0] - http://conal.net/blog/posts/semantic-editor-combinators
[1] -
http://hackage.haskell.org/packages/archive/sec/0.0.1/doc/html/Data-SemanticEditors.html
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