David,

The reason Parenscript doesn't ship with macros for this kind of thing is, I suppose, that they're so easy to write and everyone's preferences are slightly different. Since we have

(@ addr 'zipcode) => addr.zipcode

you could imagine something like this to add in the null check:

(? addr 'zipcode) => (address != null) ? address.zipcode : null

The trickiest part really is figuring out what notation you want to use. I don't know if ? is a good notation or not, it's just the first that occurred to me.

That being said, there is a structural issue with s-expressions not being so good for chaining such expressions together, since each accessor introduces a new level of nesting. This isn't readable when you have more than two or three properties in the chain:

(@ (@ (@ lottery 'winner) 'address) 'zipcode) => lottery.winner.address.zipcode

Parenscript gets around that by allowing (@ lottery 'winner 'address 'zipcode) as shorthand for (@ (@ (@ lottery 'winner) 'address) 'zipcode). By analogy, you could have (? lottery 'winner 'address 'zipcode) as shorthand for (? (? (? lottery 'winner) 'address) 'zipcode), which would add a null check before each access. But I don't see any way to represent the equivalent of lottery.drawWinner?().address?.zipcode in a flat way in s-expressions, since it's not using the same access operator all the way through.

Arguably the best solution would be to extend the PS reader with syntax for the dot operator, so lottery.winner.address.zipcode would be syntax for (@ (@ (@ lottery 'winner) 'address) 'zipcode). Then, PS being a Lisp, it would presumably not be very hard to add one's own extensions like ? for null-checking. So far, though, no one has undertaken such work on the PS reader. (Early versions of PS actually had dot syntax for property access, but not at the reader level. This wasn't convenient for many purposes, especially macro writing, since an expression like lottery.winner.address.zipcode was just one big lump of a symbol and you had to parse it yourself.)

Daniel

On Sat, Jan 19, 2013 at 6:30 PM, David Sargeant david@dsargeant.com wrote:

It would be great to be able to do "soaks" like in CoffeeScript. Example from coffeescript.org:

zip = lottery.drawWinner?().address?.zipcode

Produces

var zip, _ref;

zip = typeof lottery.drawWinner === "function" ? (_ref = lottery.drawWinner().address) != null ? _ref.zipcode : void 0 : void 0;

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