There's also the defmacro route:
(defmacro if-and [test-expr binding & tests]
`(let [~binding ~test-expr]
(and ~@(concat tests [binding]))))
Evaluates test-expr, then evaluates to the result of that if the tests all
pass, and to a falsey value otherwise (in fact, the first falsey value
returned by any of the tests). If you want only nil or the value, the and
sexp can be changed to (if (and ~@tests) binding). Short-circuits at the
first falsey test of course. Might be more convenient (fewer lambdas
needed) sometimes and more efficient sometimes -- on the other hand, to use
as a function itself it will need wrapping in a lambda: #(if-and % %
tests-using-%).
On Mon, Jun 10, 2013 at 4:31 PM, Ben Wolfson <wolf...@gmail.com> wrote:
> On Mon, Jun 10, 2013 at 1:20 PM, Steven Degutis <sbdegu...@gmail.com>wrote:
>
>> Sometimes I've wanted a function that takes a value and a bunch of tests,
>> and returns it if it passes every test, otherwise nil.
>>
>> So I wrote if-and:
>>
>> (if-and "foo"
>>
>> string?
>> #(.startsWith % "f")
>> #(.contains % "oo"))
>> ;; => "foo"
>>
>> (if-and "foo"
>> string?
>> #(.startsWith % "f")
>> #(.contains % "NOT IN THE STRING"))
>> ;; => nil
>>
>>
>> Source:
>> https://github.com/evanescence/if-and/blob/master/src/if_and/core.clj
>>
>> Does this seem useful enough to put into core?
>>
>>
> "Takes a value and a bunch of tests, and returns [the value] if it passes
> every test" implies that (if-and false false?) should return false, but:
>
> user> (if-and false false?)
> nil
>
> It's also kind of confusing to me *anyway* that (if-and false false?)
> would return false and not true (to indicate that the supplied value passed
> every test). Something like this seems more intuitive to me:
>
> user> (defn allpreds [x & ts] (every? #(% x) ts))
> #'user/allpreds
> user> (allpreds false false?)
> true
> user> (allpreds nil false?)
> false
>
> With if-and being definable in terms of allpreds:
>
> user> (defn if-and* [x & ts] (when (apply allpreds x ts) x))
> #'user/if-and*
> user> (if-and* false false?)
> false
> user> (if-and* false nil?)
> nil
>
> --
> Ben Wolfson
> "Human kind has used its intelligence to vary the flavour of drinks, which
> may be sweet, aromatic, fermented or spirit-based. ... Family and social
> life also offer numerous other occasions to consume drinks for pleasure."
> [Larousse, "Drink" entry]
>
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