Yeah, that's basically it. Keeping in mind that Haskell's data structures are immutable "let stang68 = stang67" allows to use symbol "stang68" as a new Car. Whether an actual copy is made does not matter: even if both "stang"s point to the same structure you're not allowed to change its values. El mié, 13-04-2011 a las 17:27 +0200, Christopher Done escribió:

On 13 April 2011 17:22, Andrew n marshall wrote:

Is there a syntax or function to copy a record, but with select fields populated with new values?

For example, extending LYAH's Car let stang67 = Car {company="Ford", model="Mustang", year=1967} let stang68 = stang67 `mutate` Car {year=1968}

let stang67 = Car {company="Ford", model="Mustang", year=1967} let stang68 = stang67 { year = 1968 } _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

Thanks for the follow-up. While I'm still getting used to reading/writing the numerous new operators/infix notations, I've started looking into the Data.Accessor package. Is this similar in scope to the lenses you're referring to? On 4/14/11 11:14 AM, Chaddaï Fouché wrote:

On Wed, Apr 13, 2011 at 5:22 PM, Andrew n marshall wrote:

...

Is there a syntax or function to copy a record, but with select fields populated with new values? For example, extending LYAH's Car

let stang67 = Car {company="Ford", model="Mustang", year=1967} let stang68 = stang67 `mutate` Car {year=1968}

The classic solution is simply :

...

let stang68 = stang67 {year=1968} but that is not very convenient (not at all in fact) when for instance you have several nested records or you want to base the "new" value of a field on its "old" value. Thus was invented the concept of "functional references" or "lenses" which extend this mechanism by a more complete and convenient protocol to copy a record with modification on some of its field. See

http://hackage.haskell.org/package/fclabels

http://www.cis.upenn.edu/~bcpierce/papers/newlenses-popl.pdf

There are plenty of lenses packages on Hackage, I don't know which are more used, maybe some that uses them could help here ?