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Re: Functional record “setters”

From: Noah Lavine
Subject: Re: Functional record “setters”
Date: Mon, 9 Apr 2012 21:19:18 -0400


> The goal was to have an interface close to what one would do in
> imperative programming, that is:
> = "foo";
> I think it’s quite successful at it.
> Now, I’m open for suggestions.  I don’t have any idea for a better
> interface that meets this goal.  For instance, having to spell out the
> getter names may look verbose, but I don’t see how it can be avoided.
> Any ideas?

Since updating a field requires creating a new record object (I
think), it seems useful to have an interface to update many fields at
once, to avoid creating many temporary objects. In the spirit of
brainstorming, I have two ideas to offer. First, here's an extended
version of your example:

 (let* ((p1 (set-field (person-address address-city) p "Düsseldorf"))
         (p2 (set-field (age) p1 32))))

1. Alternate a lists of field names and values. The example becomes
  (set-field p (person-address address-city) "Düsseldorf" (age) 32)

2. Pair field names and values. I think of this as analogous to
pairing cond tests and clauses. Your example becomes

  2a: (set-field p (person-address address-city "Düsseldorf"))

or maybe

  2b: (set-field p ((person-address address-city) "Düsseldorf"))

Of these, I think 2b is the most Schemelike (1 was inspired by Arc). 1
also has the disadvantage that if you leave out a list element, it
might not be immediately obvious where the problem is (although good
indentation could fix that).


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