Changes between Version 22 and Version 23 of ExtensibleRecords

Nov 17, 2007 11:45:08 PM (7 years ago)



  • ExtensibleRecords

    v22 v23  
    3737 * Permutativity:: Are `{X :: Int, Y :: Int}` and `{Y :: Int, X :: Int}` the same type? The '''Poor Man's Records''' system distinguishes these two, which makes implementation much simpler, but means that any function which accepts permuted records must be polymorphic. ''(i don't see why this is an issue? you can either list the fields you want as separate constraints, or you can have a single constraint requiring your record's type to be a permutation of the interface you want, or you can give a specific interface type and permute any record to that interface's field ordering, without ever requiring a global ordering on field types, as most other proposals do)'' 
    3838 * Repeated Fields:: Is `{X :: Int, X :: Int}` a valid record type? Both '''Poor Man's Records''' and '''Scoped Labels''' allow this type, but other systems consider this an error. ''(note that '''Poor Man's Records''' can handle both scoped and unscoped records: if you start from non-scoped records and only apply non-scoped record operations, your records will remain unscoped. you can also easily define a type predicate that guarantees your record to be unscoped. i accept, however, that a non-scoped-only style should be better supported, so i've added such a predicate for the next version of the library, and i'd like to add a type tag that turns this predicate into an invariant.)'' 
     40 Ok, opinion (since you asked for it!) :: The problem with scoping is that, in most cases, repeated fields are a programmer error, and the point of types is to catch such errors at compile time. At first sight, the ability to scope fields in this way looks like extra power to the programmer, but is this actually useful? I've seen no convincing examples where scoping allows a more clearly structured program, whereas there are plenty of cases where it will mean an error goes uncaught. If you have a good example of scoping, please add it to the examples section on this page. 
     41 .:: The problem with unpermuted records is illustrated in the example at the bottom of this page: it forces you to make functions polymorphic when they "ought" to be monomorphic. This means that no-one can use records in a straightforward way without understanding the details of all the predicates. 
     42 .:: In both cases, the usual approach (permutation, no repeats) is what most programmers expect. We have to remember that this proposal is supposed to be ''the'' records system for Haskell. It must be the right system for simple problems as well as complex ones. 
    4044= Label Namespace =