wiki:ExistingRecords

Version 7 (modified by malcolm.wallace@…, 8 years ago) (diff)

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This page is used to discuss _minor_ tweaks to the existing record system if it is decided that it will be left in basically unchanged. radical or brand new record systems should be discussed elsewhere.

  • reallow punning, Foo {x,y,z} would be interpreted as Foo {x = x, y = y, z = z} in both declaration and pattern matching contexts.
  • update syntax should not bottom out when fields are undefined. as in
data Foo = Foo { x :: String, y :: Int } | Bar { x :: String }

foo = Bar { x = "hello }

baz = foo { y = 3 }

should not result in an error, but rather pass foo 
through unchanged. update should update the record 
if it exists, but pass the type through otherwise.
This would make the update syntax actually useful

  • label-based pattern matching

the function:

  f val { x = "foo" } = 4

should match if passed a Foo or a Bar with x being equal to "foo" and val would be bound to its argument (like an @

pattern)

 g _ { y = 3 } = 4

would match only the Bar constructor since it is the only one with a y field.

This would mitigate the problems caused by accessors being partial functions since you can use a simple case statement to get the effect of an accesor that returns its result in a Maybe.

Note from Simon. I hate that the above defn of 'f' has just one argument (val {x="foo")), whereas it looks as if it has two. (This is a problem with existing Haskell.) It looks like 'f' has an argument 'val' and another arguement that is a free-standing record, something we really want in the end anyhow. Not sure how to fix this. val@{x="foo")?

  • first class update and setting syntax (more advanced, needs better syntax)

A syntax for updating and setting fields should be allowed.

some possibilites are

foo { x = }

would be equivalent to (\v -> foo { x = v })

foo { x \ }

would be equivalent to 

(\f -> foo { x = case foo of _ {x} -> foo { x = f x }; _ -> foo }) 

  • polymorphic record update

Given a record like:

data Foo a = Foo { bar :: a }

it would be nice to be able to update it like:

f = Foo { bar = 'a' }
g = f { bar = False }

Note the change in the type of the stored field. At the moment, such a record update must be written using the data constructor, not the update syntax.

open statement

having the ability to 'open' a record bringing all its values into scope would be useful for techniques such as first class modules when combined with PolymorphicComponents. a proposal is

data Record = Record { foo :: Int, bar :: String }

f :: Record -> Int
f x = ... where
   open x
   ans = ...

will desugar to

f x = ... where
   Record { foo = foo } = x 
   Record { bar = bar } = x 
   ans = ...

open x would be allowed at the top level, in a let binding, or in a where binding.