Re: Polymorphic record update

Henrik, can you please be sure this is captured on the wiki somewhere? (Sorry, I haven't checked to see if you already did that). Make yourself the owner :) peace, isaac Niklas Broberg writes:

On 1/23/06, Henrik Nilsson wrote:

[snip lots of good stuff]

This suggestion would go a long way to alleviate the burden of boiler-plate coding. It is a conservative extension, and it is intuitive at that. Indeed I believe I have written code with the suggested update mechanism many times without thinking on the type mismatch (and been beaten on my fingers by the compiler of course). :-)

...

But we'd rather not have to write such functions by hand, just as we'd rather not write update functions by hand. Maybe the record update syntax could be extended so that the function that gets generated behind the scenes only includes constructors that does NOT mention a particular field. For example, the field name(s) that must not occur could be prefixed by "~" which suggests negation in some settings. It does not have this connotation in Haskell, but at least "~" is already a special symbol. We could then write:

foo :: T a -> T Int foo x@(C1 {}) = x {f1 = 1} foo x@(C2 {}) = x {f1 = 2} foo x = x {~f1}

Now the code for "foo" only has to be changed if new constructors having a field "f1" are added.

Of course, it should be possible to combine this with the normal record update syntax. E.g.

foo :: T a -> T Int foo x@(C1 {}) = x {f1 = 1} foo x@(C2 {}) = x {f1 = 2} foo x = x {~f1, f2 = f2 x + 1}

Is this really necessary? Adding '~' seems less intuitive to me than just writing

foo :: T a -> T Int foo x@(C1 {}) = x {f1 = 1} foo x@(C2 {}) = x {f1 = 2} foo x = x

or foo x = x {f2 = f2 x + 1}

for the last example. From an implementor's point of view, if we expect the proper coercions to be inferred by the type checker it would still have to check that there are indeed no more fields than other than 'f1' that mention the parameter 'a', and also that there are no more constructors that mention 'f1'. Wouldn't it be just as simple to assert that for all the fields that mention 'a', none of these appear in any of the remaining constructors?

On the other hand pattern matching would certainly be more expressive if '~' is added, so perhaps adding it has merit of its own. If we write

foo :: T a -> T Int foo x@(C1 {}) = x {f1 = 1} foo x@(C2 {}) = x {f1 = 2} foo x = x {~f1}

there could still be more constructors in T a that do mention the 'f1' field, but there is no matching clause for them in the definition of 'foo'. But I would see that as a second separate proposal, e.g. a Proposal for Negation in Record Pattern Matching. Sure it would fit very well with the Polymorphic record update discussed here, but I would think they should be treated separately.

/Niklas _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://haskell.org/mailman/listinfo/haskell-prime