Here is the if-let*:
(defmacro if-let*
([bindings then]
`(if-let* ~bindings ~then nil))
([bindings then else]
(if (seq bindings)
`(if-let [~(first bindings) ~(second bindings)]
(if-let* ~(drop 2 bindings) ~then ~else)
~(if-not (second bindings) else))
then)))
And when-let*:
(defmacro when-let
If you have ever wished if-let and friends would allow multiple bindings
wish no more.
Check out... https://github.com/LockedOn/if-let
-Karl
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I'll chime in with my opinion on this topic.
I think the existing if-let and similar forms that have a limitation of only
allowing a single binding is a confusing restriction to place on the familiar
binding vector construct. I've always been a little uneasy about repurposing
bindi
this if-let-all do not support destructure, I writed a improved
https://gist.github.com/gfZeng/8e8e18f148d5742b064c
On Tuesday, June 9, 2015 at 8:00:53 PM UTC+8, crocket wrote:
>
> It evaluates true-case only if every local binding evaluates to true values.
> false-case has no access
nd the actual behavior of this sort of code (if it was allowed):
(if-let [a (some-a-fn)
b (some-b-fn)
c (some-c-fn)]
(some-t-fn)
(some-f-fn))
Should it short-circuit as soon as one of `a`, `b`, or `c` is falsey? Should it
evaluate all thr
At least based on my uses, I agree that this would likely bring the most
use of the now unused binding space in cores if-let. I can't think of any
useful alternatives.
Syntactically though, one could worry that the additional bindings would be
read as regular let bindings and worry abou
There's a variant of this in one of my projects as well.
If this is in "several" utility libraries *and* half the world keeps
Greenspunning versions of it in their own projects, then it might be
something that belongs in core ...
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valuates true-case only if every local binding evaluates to true values.
> false-case has no access to local bindings.
>
> (defmacro if-let-all
> "if-let-all evaluates every local binding sequentially and evaluates
> true-case only if every local binding is a truthy value.
&g
Dunaj has support for multiple bindings in if-let since version
0.5. http://www.dunaj.org/dunaj.flow.api.html#if_let
Related design page that discusses possible approaches is
at https://github.com/dunaj-project/dunaj/wiki/Conditionals
Jozef
On Tuesday, June 9, 2015 at 4:00:35 PM UTC+2, Lars
I actually wish this was how the if-let macro in core worked. Once in a
blue moon I end up writing nested if-let statements or an if-let with a
nested let. Both of these cases look so ridiculous I often re-write the
the code just avoid it.
On Tuesday, June 9, 2015 at 2:00:53 PM UTC+2
It evaluates true-case only if every local binding evaluates to true values.
false-case has no access to local bindings.
(defmacro if-let-all
"if-let-all evaluates every local binding sequentially and evaluates
true-case only if every local binding is a truthy value.
true-case has acce
Where does if-let-all serve people best? Can anyone help me find the right
clojure project to contribute to?
On Friday, June 5, 2015 at 2:44:22 PM UTC+9, crocket wrote:
>
> The macro below is called if-let-all.
>
> (defmacro if-let-all
> "if-let-all evaluates every local
Yes, if-and-let is similar to if-let-all.
On Friday, June 5, 2015 at 10:47:24 PM UTC+9, Fluid Dynamics wrote:
>
> On Friday, June 5, 2015 at 1:53:07 AM UTC-4, crocket wrote:
>>
>> Ouch, I didn't write. Gary Fredericks wrote it. I simply modified his
>> if-let-all
On Friday, June 5, 2015 at 1:53:07 AM UTC-4, crocket wrote:
>
> Ouch, I didn't write. Gary Fredericks wrote it. I simply modified his
> if-let-all macro a little bit.
>
> On Friday, June 5, 2015 at 2:44:22 PM UTC+9, crocket wrote:
>>
>> The macro below is called if
Ouch, I didn't write. Gary Fredericks wrote it. I simply modified his
if-let-all macro a little bit.
On Friday, June 5, 2015 at 2:44:22 PM UTC+9, crocket wrote:
>
> The macro below is called if-let-all.
>
> (defmacro if-let-all
> "if-let-all evaluates every local
The macro below is called if-let-all.
(defmacro if-let-all
"if-let-all evaluates every local binding sequentially and evaluates
true-case only if every local binding is a truthy value.
true-case has access to all local bindings, but false-case doesn't have access
to loca
On Wednesday, November 26, 2014 10:06:41 PM UTC-5, Michael Blume wrote:
>
> Instead of the deshadowing logic, why not
>
> (defn if-and-let*
> [bindings then-clause else-fn-name]
> (if (empty? bindings)
> then-clause
> `(if-let ~(vec (take 2 bindings))
>
s)
then-clause
`(if-let ~(vec (take 2 bindings))
~(if-and-let* (drop 2 bindings) then-clause else-fn-name)
(~else-fn-name
(defmacro if-and-let
[bindings then-clause else-clause]
(let [efname (gensym)]
`(let [~efname (fn [] ~else-clause)]
~(if-and-let* bi
Instead of the deshadowing logic, why not
(defn if-and-let*
[bindings then-clause else-fn-name]
(if (empty? bindings)
then-clause
`(if-let ~(vec (take 2 bindings))
~(if-and-let* (drop 2 bindings) then-clause else-fn-name)
(~else-fn-name
(defmacro if-and-let
Wouldn't it be nice if if-let allowed more bindings?
Try this, which I hereby dedicate into the public domain so that anyone may
use it freely in their code without restrictions:
(defn if-and-let*
[bindings then-clause else-clause deshadower]
(if (empty? bindings)
then-clause
ing form doesn't compile and I see no reason why it shouldn't:
>
> (if-let [a "a"] a (assert a))
>
> IMHO it is a bug.
>
> If anyone is of a different opinion please share.
>
> Thanks,
>
> --
> László Török
>
>
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, and can't compile.
>
>
> On Wednesday, November 19, 2014 3:34:13 PM UTC+1, Las wrote:
>>
>> Hi,
>>
>> the following form doesn't compile and I see no reason why it shouldn't:
>>
>> (if-let [a "a"] a (assert a))
>
Hi,
the following form doesn't compile and I see no reason why it shouldn't:
(if-let [a "a"] a (assert a))
IMHO it is a bug.
If anyone is of a different opinion please share.
Thanks,
--
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The docs are clear that the test occurs before the bindings:
(doc if-let)
-
clojure.core/if-let
([bindings then] [bindings then else & oldform])
Macro
bindings => binding-form test
If test is true, evaluates then with binding-form bound to the value of
test,
On Thursday, January 31, 2013 1:49:40 AM UTC+1, Sean Corfield wrote:
> but now that you've posted this, I
> can see some potential for confusion when folks first encounter
> if-let... Presumably the same confusion could arise for when-let?
>
yes, this is the confusion that
On Wednesday, January 30, 2013 8:51:47 PM UTC+1, Gary Verhaegen wrote:
> For your particular use-case, what you want is more along the lines of
>
> (if-let [errors (:password (fn-returning-errors))]
> ...)
>
yes, precisely!
mimmo
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Interesting to see this interpretation of if-let... I'd always read it
as "if the condition is truthy then let the binding be the condition
and evaluate the first expression else just evaluate the second
expression". Since the binding could create multiple named values (in
general
Perhaps I've been a little bit rude with if-let, but I really do not see how
(if-let [{erros :email} (function-returning-errors email password)]
true
false)
is not misleading. I now that the tested value is the one returning from the
function call and not the value assign
If-let does the right thing. What would your intuition expect for
(if-let [{a :a b :b} {:a 1 :b nil}]
true
false)
For your particular use-case, what you want is more along the lines of
(if-let [errors (:password (fn-returning-errors))]
...)
On Wednesday, January 30, 2013, Ben Smith
I find it helpful to view if-let as a minor variation on if, with the only
difference being that you choose to bind the results of the test-expression
to some name(s). if-let doesn't care about the values bound to the
variables named in binding-target (which might be an arbitrarily co
that means never use if-let with sequential destructoring, which brings me to
say: never use if-let, because I don't' like to remember such thing while
coding and then become crazy to catch my error because of a misleading language
feature.
mimmo
On Jan 30, 2013, at 10:32 AM
Agree with you that it is very misleading when using map-destructure in
if-let, the same applies to sequential-destructure:
user=> (if-let [[_ x] [1 nil]] true false)
true
On 13-1-30 下午5:23, "Mimmo Cosenza" wrote:
>Uhm, I do not agree.
>
>Suppose tha you have a funct
ah, compare
> (if-let [{errors :email} (function-returning-error email password)]
>true
>false)
with
(let [{errors :email) (function-returning-errros email password)]
(if errors
true
false))
I'm not saying that if-let is wrong, I'm saying I would
Uhm, I do not agree.
Suppose tha you have a function returning a map of errors (a valip validator
lib real case) like the following
{:email ["Email can't be empty"] :password ["Password can't be empty"]}
If I want to select just the email errors I would writ
>From the expansion we can see that if-let determine the result based on
the second param, in your case: {:key2 "a string"}, not the local binding
you assumed(key1), and
I think it is reasonable, for example, if we have the following code:
(if-let [{key1 key2} {:key2 "a string&q
Hi all,
I'm a little bit confused about the semantic of if-let macro.
Suppose to call it as follows with map destructoring:
(if-let [{key1 :key1} {:key2 "a string"}]
true
false))
It returns true.
But,
(let [{key1 :key1} {:key2 "a string"}]
(if key1
On Fri, Jan 4, 2013 at 2:35 PM, Michał Marczyk wrote:
> Note that if-let -- as it currently stands, I mean -- doesn't make the
> binding available to the "else" branch (so there's no way of telling
> if the init expression turned out to be false or nil). The above w
On Fri, Jan 4, 2013 at 11:59 AM, Marko Topolnik wrote:
> I do this regurarly:
>
> (-> x (fun1 arg1) (#(fun2 arg2 %)) ...)
>
> It works, but I still don't like it, I find that extra hash and parens
> distasteful.
>
>
Thanks for that idea. Don't know why I never thought to do that.
--
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ms
> weird. I would absolutely want if-let to use (and ..) on my bindings.
All bindings in "then" clause, no bindings in "else" clause. Inside
the bindings vector -- as in a regular let, that is, each init
expression sees the bindings established earlier.
Note that if-let -- as i
t it, it seemed that there
> was actually very broad consensus about the desirability of multiple
> bindings in when-let/if-let and about the behavior. Nearly everyone agreed
> it was obvious that as soon as you hit a variable that is bound to nil, you
> bail out of the express
os and on redundant
indentation due to if-let-if combinations.
> This wouldn't be such a limitation if you could use anonymous functions
> among the chained functions (since then you could set up a function with
> the "hole" anywhere you want), but this doesn
n when-let/if-let and about the behavior. Nearly everyone agreed
it was obvious that as soon as you hit a variable that is bound to nil, you
bail out of the expression, short-circuiting further evaluations and
returning nil (for when-let) or the else clause (for if-let).
There were a few quibbles
On Fri, Jan 04, 2013 at 08:58:40AM +0100, Tassilo Horn wrote:
At least in my experience, it usually matters a lot which form actually
evaluated to nil. But it's easy to write a macro `if-let-all' or so,
which would expand into
(let [x 1 y nil z 3]
(if (and x y z)
(+ x y
On Thu, Jan 03, 2013 at 11:14:30PM -0800, Evan Mezeske wrote:
Wouldn't it be more accurately named "if-and-let" if it supported that? E.g.
(if (and x y z) ...).
I can see regular if-let being useful with more than one form, just
using the last value for the conditional.
in
> Clojure
> > or like "let*" in Scheme.
> >
> > I mean e.g.:
> > (if-let* [x 1 y nil z (inc y)]
> > (+ x y z)
> > 0) ; => 0
> > ;; (inc y) shouldn't be evaluated here.
> >
> > Which means "and" doe
wrote:
> Sorry guys, I forget to mention that it should behave like "let" in Clojure
> or like "let*" in Scheme.
>
> I mean e.g.:
> (if-let* [x 1 y nil z (inc y)]
> (+ x y z)
> 0) ; => 0
> ;; (inc y) shouldn't be evaluated here.
>
> Which mean
Sorry guys, I forget to mention that it should behave like "let" in Clojure
or like "let*" in Scheme.
I mean e.g.:
(if-let* [x 1 y nil z (inc y)]
(+ x y z)
0) ; => 0
;; (inc y) shouldn't be evaluated here.
Which means "and" doesn't work there.
ly reasonable choice.
My main reason for responding is to let you know that if you really want such
behavior, macros let you roll your own without much trouble.
Andy
On Jan 3, 2013, at 10:24 PM, Edward Tsech wrote:
> Hey guys,
>
> if-let and when-let macros support only 2 forms i
Edward Tsech writes:
> java.lang.IllegalArgumentExcepdtion: if-let requires exactly 2 forms
> in binding vector(NO_SOURCE_FILE:1)
>
> Why doesn't "if-let" support any even amount of binding forms as "let"
> does?
>
> e.g.
> (if-let [x 1 y 2 z 3]
&
Wouldn't it be more accurately named "if-and-let" if it supported that?
E.g. (if (and x y z) ...).
On Thursday, January 3, 2013 10:24:57 PM UTC-8, Edward Tsech wrote:
>
> Hey guys,
>
> if-let and when-let macros support only 2 forms in binding v
Hey guys,
if-let and when-let macros support only 2 forms in binding vector:
(if-let [x 1 y 2]
...)
java.lang.IllegalArgumentExcepdtion: if-let requires exactly 2 forms in
binding vector(NO_SOURCE_FILE:1)
Why doesn't "if-let" support any even amount of binding forms as "
On Mon, May 21, 2012 at 1:43 PM, Jay Fields wrote:
> There's a few issues there, first of which is that the code doesn't
> evaluate to what's shown:
> REPL started; server listening on localhost port 21867
> user=>
> (if-let [a 1]
>(if-let [b 2]
>
There's a few issues there, first of which is that the code doesn't
evaluate to what's shown:
REPL started; server listening on localhost port 21867
user=>
(if-let [a 1]
(if-let [b 2]
(if-let [c nil]
[a b c]
[a b c])
[a b c])
[a b c])
j
Wednesday, May 16, 2012 9:16:29 AM UTC-4, Aaron Cohen wrote:
>>>
>>> Saying something is obvious and then using the word monad a paragraph
>>> later is contradictory. ;)
>>>
>>> What should happen on the else branch of the if-let; which bindings are
>
>> What should happen on the else branch of the if-let; which bindings are
>> in scope and what would be their values?
>>
>>
> None of the bindings should be available in the else branch, since there
> would be no way to know which will succeed before run-time.
>
I
Wow, the discussion continued!
I agree on what most people have said: AND-combined and none of the
bindings available in the else.
On Friday, May 18, 2012 7:20:06 AM UTC+2, FrankS wrote:
>
> Christophe Grand was "experimenting" with some extensions to if-let and
> when-le
Christophe Grand was "experimenting" with some extensions to if-let and
when-let that had implicit ANDs for the let-forms:
https://github.com/cgrand/parsley/blob/master/src/net/cgrand/parsley/util.clj
It feels intuitive to me to allow multiple if-let-forms like cgrand implements,
ural way to do it is to have the bindings
> > >> behave like the maybe monad.
> > >> [...]
>
> > > Saying something is obvious and then using the word monad a paragraph
> > later
> > > is contradictory. ;)
>
> > Hypothetically, "th
ings
> >> behave like the maybe monad.
> >> [...]
> >
> > Saying something is obvious and then using the word monad a paragraph
> later
> > is contradictory. ;)
>
> Hypothetically, "this is obvious, unlike most monads." Zing!
>
> > Wh
ave like the maybe monad.
>> [...]
>
> Saying something is obvious and then using the word monad a paragraph later
> is contradictory. ;)
Hypothetically, "this is obvious, unlike most monads." Zing!
> What should happen on the else branch of the if-let; which bindings
On Wednesday, May 16, 2012 9:16:29 AM UTC-4, Aaron Cohen wrote:
>
> Saying something is obvious and then using the word monad a paragraph
> later is contradictory. ;)
>
> What should happen on the else branch of the if-let; which bindings are in
> scope and what would be thei
Personally, I would intuitively assume that none of the bindings from
if-let would be available in the else branch.
On 16 May 2012 14:36, Moritz Ulrich wrote:
> On Wed, May 16, 2012 at 3:16 PM, Aaron Cohen wrote:
> > What should happen on the else branch of the if-let; which bindings
On May 16, 2012, at 8:16 AM, Aaron Cohen wrote:
> Saying something is obvious and then using the word monad a paragraph later
> is contradictory. ;)
If the word "monad" is scary, just pretend he said "it should short-circuit"
instead. ;)
> What should happen on
On Wed, May 16, 2012 at 3:16 PM, Aaron Cohen wrote:
> What should happen on the else branch of the if-let; which bindings are in
> scope and what would be their values?
This is kind of tricky. My opinion tends to "all bindings", but then
it's difficult to handle lazy e
On Wed, May 16, 2012 at 9:10 AM, Walter Tetzner <
robot.ninja.saus...@gmail.com> wrote:
> On Wednesday, May 16, 2012 9:01:49 AM UTC-4, Stuart Sierra wrote:How would
>> multiple bindings for if-let or when-let work? Should every binding be
>> testedd? Should they be and-e
>
> On Wednesday, May 16, 2012 9:01:49 AM UTC-4, Stuart Sierra wrote:How would
> multiple bindings for if-let or when-let work? Should every binding be
> testedd? Should they be and-ed together? Should it short-circuit if the
> first is false?
>
> I don't think th
How would multiple bindings for if-let or when-let work? Should every
binding be testedd? Should they be and-ed together? Should it short-circuit
if the first is false?
I don't think there are obvious answers to those questions.
-S
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>
> As the logical AND of all of the multiple forms? The OR? Only use the
> first expression? Only the last?
>
It should be AND.
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Note t
On Tue, May 15, 2012 at 3:47 PM, Walter Tetzner
wrote:
> On Tuesday, May 15, 2012 3:41:58 PM UTC-4, Dan Cross wrote:
>> My own personal opinion is that it makes sense in combination with 'and',
>> but others may feel differently. E.g.,
>>
>> (when-let [a (allocate-thing) b (read-into-thing a)
On May 15, 2012, at 3:15 PM, Andy Fingerhut wrote:
> If if-let/when-let had multiple bindings, how would you propose to define the
> condition of whether to do the "then" branch?
>
> As the logical AND of all of the multiple forms? The OR? Only use the first
> e
If if-let/when-let had multiple bindings, how would you propose to define the
condition of whether to do the "then" branch?
As the logical AND of all of the multiple forms? The OR? Only use the first
expression? Only the last?
I don't see that any of those is any more
On Tuesday, May 15, 2012 3:41:58 PM UTC-4, Dan Cross wrote:
>
> My own personal opinion is that it makes sense in combination with 'and',
> but others may feel differently. E.g.,
>
> (when-let [a (allocate-thing) b (read-into-thing a) c
> (extract-something-from-thing b)]
> (do-somethi
On Tue, May 15, 2012 at 3:26 PM, Aaron Cohen wrote:
> Does the principle of least surprise suggest that multiple bindings be
> combined with AND or OR?
My own personal opinion is that it makes sense in combination with 'and',
but others may feel differently. E.g.,
(when-let [a (allocate-t
x)
z (exp-3 y)]
[x y z])
would be the same as
(when-let [x (exp-1)]
(when-let [y (exp-2 x)]
(when-let [z (exp-3 y)]
[x y z])))
For the behavior of `if-let' to not be suprising given this definition
of `when-let', I think it would have to behave similarily:
(if-
Does the principle of least surprise suggest that multiple bindings be
combined with AND or OR?
--Aaron
On Tue, May 15, 2012 at 3:09 PM, Hubert Iwaniuk wrote:
> I tried using if-let with multiple binding in past as well.
> Following least surprise principle, I would like to see suppo
If if-let and when-let don't get support for multiple bindings, you could
try https://github.com/egamble/let-else .
On Tuesday, May 15, 2012 12:09:08 PM UTC-7, Hubert Iwaniuk wrote:
>
> I tried using if-let with multiple binding in past as well.
> Following least surprise principle
I tried using if-let with multiple binding in past as well.
Following least surprise principle, I would like to see support for
multiple bindings.
Cheers,
Hubert.
Vinzent <mailto:ru.vinz...@gmail.com>
May 15, 2012 5:47 PM
Or maybe if-let and when-let should support multiple bindings
Or maybe if-let and when-let should support multiple bindings, like the doc
states.
воскресенье, 13 мая 2012 г., 4:55:40 UTC+6 пользователь Borkdude написал:
>
> The docstring of if-let is as follows:
>
> bindings => binding-form test
>
> If test is true, evaluates then wi
Reasonable enough. Patch welcome.
-S
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The docstring of if-let is as follows:
bindings => binding-form test
If test is true, evaluates then with binding-form bound to the value of
test, if not, yields else
I think it should be mentioned in the docs that if-let and when-let support
only *one binding*, not multiple bindings (l
Nevermind - it was late, and I found the error message cryptic.
Sorry for throwing up a red herring.
On Jan 5, 5:30 am, Alessio Stalla wrote:
> On Wednesday, January 5, 2011 11:06:34 AM UTC+1, David wrote:
> > java.lang.String cannot be cast to clojure.lang.IFn
> > [Thrown class java.lang.Cla
On Wednesday, January 5, 2011 11:06:34 AM UTC+1, David wrote:
>
> Consider the two definitions:
>
> (defn if-let-good [str]
> (if-let [rest (seq (drop-while (partial = \a) str))]
> (first rest)
> "empty"))
>
> (defn if-let-bad [seq]
> (if-
Hello David,
what's the question ?
2011/1/5 David
> Consider the two definitions:
>
> (defn if-let-good [str]
> (if-let [rest (seq (drop-while (partial = \a) str))]
>(first rest)
> "empty"))
>
> (defn if-let-bad [seq]
> (if-let [rest (seq (dr
Consider the two definitions:
(defn if-let-good [str]
(if-let [rest (seq (drop-while (partial = \a) str))]
(first rest)
"empty"))
(defn if-let-bad [seq]
(if-let [rest (seq (drop-while (partial = \a) seq))]
(first rest)
"empty"))
The only difference betw
Looks like its already done:
https://www.assembla.com/spaces/clojure/tickets/103-gc-issue-99--incorrect-error-with-if-let
On 20 October 2010 22:20, David Jagoe wrote:
> Hi all,
>
> I noticed today (clojure 1.2) that using if-let incorrectly thusly:
>
> (if-let [a 1]
> (printl
Hi all,
I noticed today (clojure 1.2) that using if-let incorrectly thusly:
(if-let [a 1]
(println "1")
(println "2")
(println "3"))
Instead of e.g.
(if-let [a 1]
(println "1")
(do
(println "2")
(println "3")))
Res
On Jun 7, 6:37 am, toddg wrote:
> I'm attempting to read and understand core.clj, and I'm walking
> through methods as I run into them, trying to understand them, line by
> line. I' mostly understand read-lines (w/ the exception of the last
> line), but I do not follo
I'm attempting to read and understand core.clj, and I'm walking
through methods as I run into them, trying to understand them, line by
line. I' mostly understand read-lines (w/ the exception of the last
line), but I do not follow lazy-seq or if-let. Could someone check my
deconst
> (let [basedir (if-let [bdir (:basedir *locs)] bdir ".")]
> ...)
I'd personally write that as:
(let [basedir (or (:basedir *locs*) ".")]
...)
There is also when-let, which can be used to iterate over sequences:
(loop [items some-sequence]
(when-l
,
>
> Am 08.02.2009 um 15:47 schrieb Adrian Cuthbertson:
>
>> Here's one, I'm setting basedir to either :basedir in a map in *locs
>> (a thread-local var) or to "." if :basedir was not found in the map...
>>
>> (let [basedir (if-let [bdir (:basedir *loc
Hi,
Am 08.02.2009 um 15:47 schrieb Adrian Cuthbertson:
Here's one, I'm setting basedir to either :basedir in a map in *locs
(a thread-local var) or to "." if :basedir was not found in the map...
(let [basedir (if-let [bdir (:basedir *locs)] bdir ".")]
...)
Here's one, I'm setting basedir to either :basedir in a map in *locs
(a thread-local var) or to "." if :basedir was not found in the map...
(let [basedir (if-let [bdir (:basedir *locs)] bdir ".")]
...)
i.e bdir assumes the value of the test and if that is
Hi,
Am 08.02.2009 um 15:25 schrieb Mark Volkmann:
Can someone show me an example of a good use of if-let?
One use:
(defn some-function
[& args]
(if-let [fst (first args)]
(do-something-with fst)
(do-something-else)))
Sincerely
Meikel
smime.p7s
Description: S/
Can someone show me an example of a good use of if-let?
I find its doc string a little confusing. It says "If test is true,
evaluates then with binding-form bound to the value of test, if not,
yields else". However, it doesn't have a parameter named "test". I
assume
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