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, James Xu <xumingming64398...@gmail.com> wrote:
> 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" <mimmo.cose...@gmail.com> wrote:
>
>> 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
>>>>
>>>>
>>>> --
>>>> --
>>>> You received this message because you are subscribed to the Google
>>>> Groups "Clojure" group.
>>>> To post to this group, send email to clojure@googlegroups.com
>>>> Note that posts from new members are moderated - please be patient with
>>>> your first post.
>>>> To unsubscribe from this group, send email to
>>>> clojure+unsubscr...@googlegroups.com
>>>> For more options, visit this group at
>>>> http://groups.google.com/group/clojure?hl=en
>>>> ---
>>>> You received this message because you are subscribed to the Google
>>>> Groups
>>>> "Clojure" group.
>>>> To unsubscribe from this group and stop receiving emails from it, send
>>>> an
>>>> email to clojure+unsubscr...@googlegroups.com.
>>>> For more options, visit https://groups.google.com/groups/opt_out.
>>>>
>>>>
>>>
>>>
>>> --
>>> --
>>> You received this message because you are subscribed to the Google
>>> Groups "Clojure" group.
>>> To post to this group, send email to clojure@googlegroups.com
>>> Note that posts from new members are moderated - please be patient with
>>> your first post.
>>> To unsubscribe from this group, send email to
>>> clojure+unsubscr...@googlegroups.com
>>> For more options, visit this group at
>>> http://groups.google.com/group/clojure?hl=en
>>> ---
>>> You received this message because you are subscribed to the Google
>>> Groups "Clojure" group.
>>> To unsubscribe from this group and stop receiving emails from it, send
>>> an email to clojure+unsubscr...@googlegroups.com.
>>> For more options, visit https://groups.google.com/groups/opt_out.
>>>
>>>
>>
>> --
>> --
>> You received this message because you are subscribed to the Google
>> Groups "Clojure" group.
>> To post to this group, send email to clojure@googlegroups.com
>> Note that posts from new members are moderated - please be patient with
>> your first post.
>> To unsubscribe from this group, send email to
>> clojure+unsubscr...@googlegroups.com
>> For more options, visit this group at
>> http://groups.google.com/group/clojure?hl=en
>> ---
>> You received this message because you are subscribed to the Google Groups
>> "Clojure" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to clojure+unsubscr...@googlegroups.com.
>> For more options, visit https://groups.google.com/groups/opt_out.
>>
>>
>
>
> --
> --
> You received this message because you are subscribed to the Google
> Groups "Clojure" group.
> To post to this group, send email to clojure@googlegroups.com
> Note that posts from new members are moderated - please be patient with your
> first post.
> To unsubscribe from this group, send email to
> clojure+unsubscr...@googlegroups.com
> For more options, visit this group at
> http://groups.google.com/group/clojure?hl=en
> ---
> You received this message because you are subscribed to the Google Groups
> "Clojure" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to clojure+unsubscr...@googlegroups.com.
> For more options, visit https://groups.google.com/groups/opt_out.
>
>
--
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
---
You received this message because you are subscribed to the Google Groups
"Clojure" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to clojure+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/groups/opt_out.
