[Haskell-cafe] Return of the revenge of the revisit of the extensible records, reiterated

oleg at okmij.org oleg at okmij.org
Thu Nov 28 11:44:51 UTC 2013

First of all, I should point out that faster extensible records have 
already been developed, within the HList framework:

        Just Do It While Compiling!
        Fast Extensible Records In Haskell

The authors did it without needing the total order on
labels. However, one of the encoding of labels described in the HList
paper had total order. I guess we weren't motivated enough to exploit

> The real benefit of keeping the row sorted is that { x = 0 , y = 0 } and {
> y = 0, x = 0 } have the same type. When a row is not sorted, as in HList,
> then if we for example have an instance Eq for a row (because all elements
> support Eq) then for using (==) both arguments would have to the same order
> in the row or we need a manual call to a permutation function. When keeping
> the row ordered, this is not necessary. The same kind of problem
> occurs
> The same kind of problem occurs when we fix the type of a function to
> a specific row: ( using whishful syntax ) 
> f :: Rec [ x = Int , y = Int] -> Int
> If the row is not ordered, then f { y = 0 , x = 0 } will not typecheck and
> will require a manual call to permute the row.

This is all very true. However, if we wish to pass the function f
above the record {y=0, x=0} (with permuted fields), we most likely
wish to pass that function a record {x=0, y=0, z='a'} with more
fields. Most of the time when we deal with extensible records, we
really wish to explore their extensibility. Keeping fields sorted
does not help at all in the latter problem -- we must manually insert
the call to the subtyping coercion function. Once we do that, the
problem with the order of the fields disappears.

I also would like to point out that there are two sorts of
record types. One sort is
        Rec [x: Int, y:Bool]
in the imagine syntax. Current HList types are uglier versions of the
above. But there is another sort:
        (HasField r x Int, HasField r y Bool) => r
It represents an extensible record type. Extensibility is build in,
and the order of the fields is immaterial. Quite a few functions on
records can be given the above type. Furthermore, the second sort can
be used not only with structural subtyping but also with nominal

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