On 15 February 2017 at 23:06 Harendra Kumar wrote:
On 16 February 2017 at 02:31, Anthony Clayden wrote:
I am aware of the ghc "overloaded records" proposal. rawr provides anonymous extensible records using the overloaded labels feature of ghc 8. Records can be merged or partitioned. I believe, the key difference between "overloaded records" and rawr is that the latter provides extensible records while the former does not.
Yes. But you seem to be talking about a different 'feature' of rawr. For gathering params to a function, I see no need for extensible records. The params are known at compile time. If for a DSL you want 'optional' fields, make them Maybe's, with a default of Nothing.
I am trying to write a program which provides a friendly high level DSL to the user.
'user' here seems to mean programmer, if you're looking for static guarantees.
I want a pure function like API but instead of passing positional parameters I want the user to be able to specify arguments based on keywords and be able to skip any optional arguments. Something like the following, name is mandatory and email is optional:
maintainer (#name := "Harendra Kumar", #email := "xyz@gmail.com")
I can achieve this using rawr. The argument to the function is an anonymous record ...
OK. But I'm not seeing why it has to be anonymous. Again, if it's for a specific function, it needs specific parameters. An anonymous record might provide keywords not valid, as well as fail to provide keywords expected.
and we can pattern match partially using the mandatory fields in the record to statically check that those fields are present.
OK. That's the critical feature you want, then. It wasn't clear from your O.P.
I was only trying to say that this is a pretty useful feature in python and I guess in some other imperative languages too.
(I don't think it's anything to do with programming paradigm.)
It allows you to write self documenting code where necessary.
It will be nice if we have a way to achieve something like
No the code you posted was not self-documenting. (The comments were useful, thank you.) Only a python-charmer would claim code full of graphic chars is 'self-documenting'. I have no idea what it was that made #name mandatory but #email not. this.
There's nothing in the standard language currently.
[example]
This is the first approach that I tried, this is commonly used in many libraries. The only drawback with this is that I cannot enforce mandatory/optional fields statically. All fields are optional in this case.
So your DSL is going to be compiled? Not intepreted? Including putting in values like #name? I'd expect #name would come from end-user input at run time(?) Yes the approach I illustrated is common in many libraries. Perhaps authors of those libraries are listening in, and could comment. AntC