On Tue, 14 Feb 2017 at 18:50, Harendra Kumar said:
In my application, I want some of the fields in a record to have default values while others must always be specified by the user. I can implement this using the rawr records library (http://hackage.haskell.org/package/rawr)
Hi Harendra. I believe rawr builds on some of the work in 'overloaded records'. That's a suite of features being developed right now look here for what's coming soon https://github.com/adamgundry/ghc-proposals/blob/overloaded-record-fields/pr...
like so:
def :: ("b" := String) -> R ("a" := Int, "b" := String) def t = (R (#a := 0) :*: R t) -- merge with defaults
let r = def (#b := "hello")
That looks like some sort of anonymous/extensible records system. What is data constructor `R`; where is it defined? There's a few mentions of 'anonymous' records on that link above. (And mention of rawr.) It's not clear what you're trying to do. Do you need anonymous/extensible records? Why use `let` to bind a value for `r`?
Here field "a" has a default value (i.e. it is optional when "def" is used) but field "b" does not have a default value and has to be explicitly specified by the user. This is made possible by record merging feature of rawr, making the record extensible.
I could not find an equivalent way of achieving this using overloaded records (http://hackage.haskell.org/package/overloaded-records).
So far, overloaded records is based on standard-ish Haskell record decls. So you must declare a distinct record constructor and field labels. Like data MyR = MyR { a :: Int, b :: String }; If you want default values: myRdef = MyR{ a = 0 }; -- don't have to give b Then bind some value, to incorp defaults. r = myRdef { b = "hello" }; -- takes the defult for a
Is record merging possible or can be made possible
using overloaded-records or with the ghc overloaded records poposal? If not, is there a plan to make something like this
It looks to me like you don't need '"record merging". What do you think it is; why do you think you need it? possible in future? I'd say it's on the radar, but a long way off.
Or will this never be possible with the in built record syntax?
Note that, this is handy to simulate mandatory and
I think that built-in record syntax will fit your requirements. optional keyword
arguments to a function by passing them in the form of a record.
Something which is available in other languages like
Sure. Keyword arguments to a function are fixed. No need for building dynamic/anonymous structures. Yes it's good (and standard practice) to pass as a record. So you can use a standard built-in Haskell record decl, as I used above. python. It looks to me that you're trying to write Python code in Haskell. Not a good idea. Use built-in/static features first. Think about the difference between a compiled vs interpreted language. Only when static features really really aren't working, consider dynamic/extensible. AntC