Best CLISP idiom for loop, test, return

[hjellinek]

@rmkaplan and co.

I've got a few situations in which I'm looping over the elements of a list and want to call a predicate on each element. If true, I want to exit the loop with a value obtained by calling another function on the i.s.var.

Here's what I came up with. I'd like to avoid the explicit call to RETURN:

(for DIR DATA in MYDIRECTORIES when (SETQ DATA (READ-DATA DIR))
    do (RETURN (OPERATE-ON DATA))

thereis is almost what I want, but it makes the expression return the value of the i.s.var (DIR). I'd rather not access CLISP internal vars like $$VAL and friends in my code.

Is there a more concise or idiomatic way to do this?

[masinter]

why avoid the RETURN? If you don't like the anonymous return you can use a named block instead of the implicit one.

(CL:BLOCK DATA-RETURN (for DIR DATA in ... do (RETURN-FROM DATA-RETURN (OPERATE-ON DATA))))

[hjellinek]

It just seems a shame to use a non-CLISP construct when thereis is so close to what I want. To me, it's like the RETURN is breaking the fourth wall.

[nbriggs]

$$VAL is a documented way of getting at that -- e.g., in the IRM for the LARGEST/SMALLEST i.s.operator --

"Returns the value of the i.v. that provides the smallest value of FORM. $$EXTREME is always bound to the current greatest/smallest value, $$VAL to the value of the i.v. from which it came."

and in 9.8.5 Miscellaneous Hints on I.S.Oprs --

…

While the exact form of the translation of an iterative statement depends on which operators are present, a PROG will always be used whenever the i.s. specifies dummy variables, i.e., if a BIND operator appears, or there is more than one variable specified by a FOR operator, or a GO, RETURN, or a reference to the variable $$VAL appears in any of the operands.

-- Nick

[nbriggs]

9.8.5 also notes that

The user can also GO to $$LP, $$ITERATE, or $$OUT, or explicitly set $$VAL.

And it also uses an example with finally (RETURN ...) -- so that's really an expected pattern of use.

[hjellinek]

I'm OK with my original formulation and will keep it.