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 write something like that
(if-let [{errors :email} (function-returning-error email password)]
true
false)
Reading the above code you're led to believe that if there are email errors,
errors local binding will be true. Instead, it returns true even if the are no
email errors but there are password errors and you never get the false branch.
An if you want to catch password errors you would write something like
(if-let [{errors :password} (function-returning-errors email password)]
true
false)
In either case you never get the false branch when function-returning-errors
return an error which is not the one you're looking for
Mimmo
On Jan 30, 2013, at 10:05 AM, James Xu <xumingming64398...@gmail.com> wrote:
> 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"}]
> true
> false))
>
>
> Should if-let determine the result based on key1? key2? IMO {key1 key2} in
> a whole is more reaonable. And {key1 key2} == {:key2 "a string"}, then the
> result is true.
>
>
>
> On 13-1-30 下午4:51, "Mimmo Cosenza" <mimmo.cose...@gmail.com> wrote:
>
>> 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
>> true
>> false))
>>
>> returns false.
>>
>>
>> The macro expansion of the former explains why
>>
>> (macroexpand-1 '(if-let [{key1 :key1} {:key2 "a string"}] true false))
>>
>> returns
>> (clojure.core/let [temp__3971__auto__ {:key2 "a string"}] (if
>> temp__3971__auto__ (clojure.core/let [{key1 :key1} temp__3971__auto__]
>> true) false))
>>
>> the consequence, IMHO, is that I would never suggest to use map
>> restructuring inside and if-let binding, because the syntax let me think
>> the result its the opposite of its semantic,
>>
>> Am I completely wrong?
>>
>> mimmo
>>
>>
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