record update

Jonathan Geddes

11 Sep 2010 11 Sep '10

1:21 p.m.

I know that record updates is a topic that has become a bit of a dead horse, but here I go anyway: I find that most of the record updates I read and write take the form

...

someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 = f $ field1 myRecord , field2 = g $ field2 myRecord , field3 = h $ filed3 myRecord }

I find myself wishing I could write something more like

...

someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 => f , field2 => g , field3 => h }

with equivalent semantics. Here => reads "is transformed by". Operator = could still be used for assignment as in current record updates. The best part about such an extension, in my opinion, is that it would open the door for anonymous lambda record updates. Something like:

...

someUpdate :: MyRecord -> MyRecord someUpdate = \{field1 => f, field2 => g, field3 => h}

again, with the same semantics. This becomes possible because you no longer need to refer to the record within the {} part of the update. This would be useful, for example, in the State monad. We could write:

...

someStateTransform :: State MyRecord () someStateTransform = do modify $ \{field1 => (++"!")} ...

where currently we see code like

...

someStateTransform :: State MyRecord () someStateTransform = do modify $ \record->record{field1 = (++"!") $ field1 record} ...

which repeats the record name 3 times and the field name twice. The repetition just feels out of place next to all the other terse, readable Haskell code in the program. So what do my fellow haskellers think? Is this idea worth writing up a proposal for? Alternatively, can you offer me some advice on writing code in Haskell 2010 that avoids the ugly, repetitive style of record update? --Jonathan

Show replies by date

Henning Thielemann

11 Sep 11 Sep

1:53 p.m.

Jonathan Geddes schrieb:

...

I know that record updates is a topic that has become a bit of a dead horse, but here I go anyway:

I find that most of the record updates I read and write take the form

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 = f $ field1 myRecord , field2 = g $ field2 myRecord , field3 = h $ filed3 myRecord }

I find myself wishing I could write something more like

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 => f , field2 => g , field3 => h }

data-accessor and similar packages may become your friends. data-accessor allows you to write: someUpdate = (field1 ^: f) . (field2 ^: g) . (field3 ^: h)

Jonathan Geddes

3:27 p.m.

On Sat, Sep 11, 2010 at 11:53 AM, Henning Thielemann wrote:

...

Jonathan Geddes schrieb:

...
I know that record updates is a topic that has become a bit of a dead horse, but here I go anyway:

I find that most of the record updates I read and write take the form

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 = f $ field1 myRecord , field2 = g $ field2 myRecord , field3 = h $ filed3 myRecord }

I find myself wishing I could write something more like

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 => f , field2 => g , field3 => h }

data-accessor and similar packages may become your friends.

data-accessor allows you to write:

someUpdate = (field1 ^: f) . (field2 ^: g) . (field3 ^: h)

data-accessor is a pretty cool package, but if I understand correctly, your "fields" are not the same as the straight functions you get from defining a record, and cant' be used as regular functions. So you have to create these Data.Accessor.Accessors. Is defining accessors really any better than just writing update functions like the following?

...

updateField1 :: (Field1Type -> Field1Type) -> MyRecord -> MyRecord updateField1 f x = x{field1 = f $ field1 x} someUpdate = (updateField1 f) . j(updateField2 g) . (updateField3 h)

I understand that there is a package data-accessor-template for generating accessors, but couldn't you use TH for generating updater functions as well? It seems like something as fundamental as record update should have a clean, build-in syntax. Or am I thinking too imperatively? --Jonathan

Henning Thielemann

3:42 p.m.

On Sat, 11 Sep 2010, Jonathan Geddes wrote:

...

On Sat, Sep 11, 2010 at 11:53 AM, Henning Thielemann

...
data-accessor and similar packages may become your friends.

data-accessor allows you to write:

someUpdate = (field1 ^: f) . (field2 ^: g) . (field3 ^: h)

data-accessor is a pretty cool package, but if I understand correctly, your "fields" are not the same as the straight functions you get from defining a record,

right

...

and cant' be used as regular functions.

right

...

So you have to create these Data.Accessor.Accessors.

right

...

Is defining accessors really any better than just writing update functions like the following?

It's just, that you can use the same name for all kinds of access (set, get, modify) and have nice combinators.

...

...
updateField1 :: (Field1Type -> Field1Type) -> MyRecord -> MyRecord updateField1 f x = x{field1 = f $ field1 x} someUpdate = (updateField1 f) . j(updateField2 g) . (updateField3 h)

I understand that there is a package data-accessor-template for generating accessors, but couldn't you use TH for generating updater functions as well?

Sure. But what about names? You would need distinct names for 'get' (like 'field1') and 'modify' (like 'updateField1'), but then you can also write update field1 (or (modify field1) as in data-accessor)

...

It seems like something as fundamental as record update should have a clean, build-in syntax. Or am I thinking too imperatively?

I think accessors are even more functional than the built-in record syntax. You can nicely combine them with Category's '.' operator in order to access fields in sub-records. It would be cool, if the field identifiers of Haskell records would be Accessors and not just Getters. This would be a recursive problem, since Accessor is itself a record. However, field names could represent functions of type record -> (field, field -> record) .

Stephen Tetley

3:36 p.m.

On 11 September 2010 18:21, Jonathan Geddes wrote:

...

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 = f $ field1 myRecord , field2 = g $ field2 myRecord , field3 = h $ filed3 myRecord }

Applicatively, using no additional libraries, is how I do it: updateAllThree :: MyRecord -> MyRecord updateAllThree = (\s a b c -> s { field1 = f a, field2 = g b, field3 = h c}) <*> field1 <*> field2 <*> field3 Note - (<$>) is not used, and the left hand function has one extra argument. This style exploits the fact that (<*>) is the Startling (S) combinator. Unfortunately, I haven't worked out how to do nested records in this style.

Henning Thielemann

3:45 p.m.

On Sat, 11 Sep 2010, Stephen Tetley wrote:

...

On 11 September 2010 18:21, Jonathan Geddes wrote:

...
...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 = f $ field1 myRecord , field2 = g $ field2 myRecord , field3 = h $ filed3 myRecord }

Applicatively, using no additional libraries, is how I do it:

updateAllThree :: MyRecord -> MyRecord updateAllThree = (\s a b c -> s { field1 = f a, field2 = g b, field3 = h c}) <*> field1 <*> field2 <*> field3

Cute!

...

Note - (<$>) is not used, and the left hand function has one extra argument. This style exploits the fact that (<*>) is the Startling (S) combinator.

It uses the Applicative instance for functions.

Stephen Tetley

3:47 p.m.

On 11 September 2010 20:45, Henning Thielemann wrote:

...

It uses the Applicative instance for functions.

Yep - I meant that, but somehow didn't write it, then pressed the send button...

Wolfgang Jeltsch

13 Sep 13 Sep

8:24 a.m.

Am Samstag, den 11.09.2010, 11:21 -0600 schrieb Jonathan Geddes:

...

I know that record updates is a topic that has become a bit of a dead horse, but here I go anyway:

I find that most of the record updates I read and write take the form

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 = f $ field1 myRecord , field2 = g $ field2 myRecord , field3 = h $ filed3 myRecord }

I find myself wishing I could write something more like

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 => f , field2 => g , field3 => h }

with equivalent semantics. Here => reads "is transformed by". Operator = could still be used for assignment as in current record updates.

The best part about such an extension, in my opinion, is that it would open the door for anonymous lambda record updates. Something like:

...
someUpdate :: MyRecord -> MyRecord someUpdate = \{field1 => f, field2 => g, field3 => h}

again, with the same semantics. This becomes possible because you no longer need to refer to the record within the {} part of the update.

This would be useful, for example, in the State monad. We could write:

...
someStateTransform :: State MyRecord () someStateTransform = do modify $ \{field1 => (++"!")} ...

where currently we see code like

...
someStateTransform :: State MyRecord () someStateTransform = do modify $ \record->record{field1 = (++"!") $ field1 record} ...

which repeats the record name 3 times and the field name twice. The repetition just feels out of place next to all the other terse, readable Haskell code in the program.

So what do my fellow haskellers think? Is this idea worth writing up a proposal for?

Alternatively, can you offer me some advice on writing code in Haskell 2010 that avoids the ugly, repetitive style of record update?

--Jonathan

You might want to have a look at the records package: http://hackage.haskell.org/package/records Here is a code example:

...

import Data.Kind import Data.TypeFun import Data.Record import Data.Record.Combinators

data Surname = Surname deriving (Show) data Age = Age deriving (Show) data Room = Room deriving (Show)

instance Name Surname where name = Surname instance Name Age where name = Age instance Name Room where name = Room

oldData = X :& Surname := "Jeltsch" :& Age := 31 :& Room := "EH/202" `withStyle` Id KindStar

newData = modify (X :& Age := (+2) :& Room := const "HG/2.39") oldData

Evaluating newData gives you:

...

X :& Surname := "Jeltsch" :& Age := 33 :& Room := "HG/2.39"

If you have any question regarding the records package, please ask, since I’m its author. :-) Best wishes, Wolfgang

Chris Eidhof

10:57 a.m.

For completeness, using fclabels (yet another record package) you can write it like this:

...

{-# LANGUAGE TemplateHaskell #-} module Records where

import Data.Record.Label

data MyRecord = MyRecord { _field1 :: String, _field2 :: Int, _field3 :: Bool }

$(mkLabels [''MyRecord])

modifyThree f g h = modL field1 f . modL field2 g . modL field3 h

-chris On 11 sep 2010, at 19:21, Jonathan Geddes wrote:

...

I know that record updates is a topic that has become a bit of a dead horse, but here I go anyway:

I find that most of the record updates I read and write take the form

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 = f $ field1 myRecord , field2 = g $ field2 myRecord , field3 = h $ filed3 myRecord }

I find myself wishing I could write something more like

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 => f , field2 => g , field3 => h }

with equivalent semantics. Here => reads "is transformed by". Operator = could still be used for assignment as in current record updates.

The best part about such an extension, in my opinion, is that it would open the door for anonymous lambda record updates. Something like:

...
someUpdate :: MyRecord -> MyRecord someUpdate = \{field1 => f, field2 => g, field3 => h}

again, with the same semantics. This becomes possible because you no longer need to refer to the record within the {} part of the update.

This would be useful, for example, in the State monad. We could write:

...
someStateTransform :: State MyRecord () someStateTransform = do modify $ \{field1 => (++"!")} ...

where currently we see code like

...
someStateTransform :: State MyRecord () someStateTransform = do modify $ \record->record{field1 = (++"!") $ field1 record} ...

which repeats the record name 3 times and the field name twice. The repetition just feels out of place next to all the other terse, readable Haskell code in the program.

So what do my fellow haskellers think? Is this idea worth writing up a proposal for?

Alternatively, can you offer me some advice on writing code in Haskell 2010 that avoids the ugly, repetitive style of record update?

--Jonathan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-Steffen

14 Sep 14 Sep

1:10 p.m.

While we are at it using Semantic Editor Combinators (sec on hackage):

...

{-# LANGUAGE TemplateHaskell #-}

module T where

import Data.SemanticEditors

data MyRecord = MyRecord { field1 :: String, field2 :: Int, field3 :: Bool } deriving(Show)

mkEditors [''MyRecord]

editRecord str = (editField1.set) newName -- set field1 to new value . editField3 not -- apply function (not) to field3 . (editIf field3.editField2.editIf (<10)) (1+) -- increase field2's value if field2's value < 10 -- and field3 is True

sec also supports functions, lists, Maybe and other monads Chris Eidhof wrote:

...

For completeness, using fclabels (yet another record package) you can write it like this:

...
{-# LANGUAGE TemplateHaskell #-} module Records where

import Data.Record.Label

data MyRecord = MyRecord { _field1 :: String, _field2 :: Int, _field3 :: Bool }

$(mkLabels [''MyRecord])

modifyThree f g h = modL field1 f . modL field2 g . modL field3 h

-chris

On 11 sep 2010, at 19:21, Jonathan Geddes wrote:

...
I know that record updates is a topic that has become a bit of a dead horse, but here I go anyway:

I find that most of the record updates I read and write take the form

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 = f $ field1 myRecord , field2 = g $ field2 myRecord , field3 = h $ filed3 myRecord }

I find myself wishing I could write something more like

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord = myRecord { field1 => f , field2 => g , field3 => h }

with equivalent semantics. Here => reads "is transformed by". Operator = could still be used for assignment as in current record updates.

The best part about such an extension, in my opinion, is that it would open the door for anonymous lambda record updates. Something like:

...
someUpdate :: MyRecord -> MyRecord someUpdate = \{field1 => f, field2 => g, field3 => h}

again, with the same semantics. This becomes possible because you no longer need to refer to the record within the {} part of the update.

This would be useful, for example, in the State monad. We could write:

...
someStateTransform :: State MyRecord () someStateTransform = do modify $ \{field1 => (++"!")} ...

where currently we see code like

...
someStateTransform :: State MyRecord () someStateTransform = do modify $ \record->record{field1 = (++"!") $ field1 record} ...

which repeats the record name 3 times and the field name twice. The repetition just feels out of place next to all the other terse, readable Haskell code in the program.

So what do my fellow haskellers think? Is this idea worth writing up a proposal for?

Alternatively, can you offer me some advice on writing code in Haskell 2010 that avoids the ugly, repetitive style of record update?

--Jonathan _______________________________________________ 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

-- View this message in context: http://old.nabble.com/record-update-tp29686064p29710821.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

Jonathan Geddes

3:31 p.m.

Wow, I had no idea there were so many record packages! This indicates a couple things to me: a) Haskell is very flexible. b) I'm not the only one who things the built-in record system isn't perfect. Digging a bit deeper, it looks like some of the record-related ghc extensions might also be useful, such as record punning and field disambiguation. Since these are already extensions, they're more likely to make it into Haskell 20XX. Are these considered to be the solution to current record syntax problems? With these extensions, couldn't I write the following?

...

someUpdate :: MyRecord -> MyRecord someUpdate myRecord@(MyRecord{..}) = let { field1 = f field1 , field2 = g field2 , field3 = h filed3 } in myRecord{..}

Conrad Parker

6:28 p.m.

On 15 September 2010 04:31, Jonathan Geddes wrote:

...

Wow, I had no idea there were so many record packages! This indicates a couple things to me: a) Haskell is very flexible. b) I'm not the only one who things the built-in record system isn't perfect. Digging a bit deeper, it looks like some of the record-related ghc extensions might also be useful, such as record punning and field disambiguation. Since these are already extensions, they're more likely to make it into Haskell 20XX. Are these considered to be the solution to current record syntax problems? With these extensions, couldn't I write the following?

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord@(MyRecord{..}) = let { field1 = f field1 , field2 = g field2 , field3 = h filed3 } in myRecord{..}

or just: someUpdate :: MyRecord -> MyRecord someUpdate myRecord@MyRecord{..} = myRecord{ field1 = f field1 , field2 = g field2 , field3 = h field3 } Conrad.

Luke Palmer

7:17 p.m.

On Tue, Sep 14, 2010 at 1:31 PM, Jonathan Geddes wrote:

...

With these extensions, couldn't I write the following?

...
someUpdate :: MyRecord -> MyRecord someUpdate myRecord@(MyRecord{..}) = let { field1 = f field1 , field2 = g field2 , field3 = h filed3 } in myRecord{..}

No, those are recursive let bindings! If f = (1:), then field1 = [1,1,1,1...] As Conrad suggests, use: someUpdate myRecord@(MyRecord{..}) = myRecord { field1 = f field1 , field2 = f field2 , field3 = f field3 } The reason this works is that "field1" in "field1 = " is not a real scoped variable, but rather an identifier for a field in the record. It's all somewhat subtle... Luke

Wolfgang Jeltsch

1 Oct 1 Oct

4:05 a.m.

Am Dienstag, den 14.09.2010, 13:31 -0600 schrieb Jonathan Geddes:

...

Wow, I had no idea there were so many record packages!

The trouble is that only one of them (i.e., mine) is categorized under “Records” on Hackage. Best wishes, Wolfgang

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-Steffen
Chris Eidhof
Conrad Parker
Henning Thielemann
Henning Thielemann
Jonathan Geddes
Luke Palmer
Stephen Tetley
Wolfgang Jeltsch

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