# [erlang-questions] Illegal map key in pattern

Per Hedeland per@REDACTED
Mon Aug 5 13:11:36 CEST 2019

```On 2019-08-05 12:28, Valentin Micic wrote:
>> On 05 Aug 2019, at 11:03, Per Hedeland <per@REDACTED> wrote:
>>
>> On 2019-08-05 09:29, Raimo Niskanen wrote:
>>> On Sat, Aug 03, 2019 at 06:14:15PM +0200, Valentin Micic wrote:
>>>> Hi,
>>>>
>>>> I am a bit nonplussed with the behaviour of map module&consider the following map:
>>>>
>>>>
>>>> f(MAP), MAP = #{ {12,1} => "some value", {12,2} => "some other value"}.
>>>>
>>>>
>>>> Then, the following works:
>>>>
>>>>
>>>> f(V), case MAP of #{ {1,12} := V} -> V end.
>>>>                              "some value"
>>>>
>>>>
>>>>
>>>> Then, surely, a snippet below should also work... except that it doesnt:
>>>>
>>>>
>>>>   f(DCID), f(SORD), DCID = 12, SORD=1.
>>>>
>>>> f(V), case MAP of #{ {DCID, SORD} := V} -> V end.
>>>>
>>>> * 1: illegal map key in pattern
>>> The syntax of your examples makes no sense to me, so I take a shot in the
>>
>> It's just "shell syntax", try it:-) - the f/1 is the shell's "forget"
>> function, to ensure that the variable is unbound.
>>
>>>      fun (K, V, Map) ->
>>> 	case Map of
>>> 	    #{ K := V} -> V
>>> 	end
>>>      end.
>>>      fun (K, V, Map) ->
>>> 	case Map of
>>> 	    #{ {K} := V} -> V
>>> 	end
>>>      end.
>>>      fun (K, V, Map) ->
>>> 	K_1 = {K},
>>> 	case Map of
>>> 	    #{ K_1 := V} -> V
>>> 	end
>>>      end.
>>> Try those in the shell and you will find out that the first produces a
>>> fun(), the second barfs with "illegal map key in pattern", and the third
>>> produces a fun().
>>> From:
>>>      http://erlang.org/doc/reference_manual/expressions.html#map-expressions
>>> section "Maps in Patterns"
>>>      Matching of key-value associations from maps is done as follows:
>>>      #{ K := V } = M
>>>      Here M is any map. The key K must be an expression with bound
>>>      variables or literals. V can be any pattern with either bound
>>>      or unbound variables.
>>> So, the key must be a literal or a bound variable.
>>
>> Hm, that's not what the documentation that you quote says - surely, if
>> K is a bound variable, {K} is "an expression with bound variables", as
>> is {DCID, SORD} in Valentin's case. And of course there's no way the
>> variableS can be plural if not combined into an expression somehow...
>>
>>> It's a known limitation.
>>
>> So it seems the documentation is wrong, and should drop the
>> "expression" and plurals: "The key K must be a bound variable or a
>> literal." (as you said).
>>
>> --Per
>
> Thank you Per for articulating my issue in a way I wasnt able to do. :-)
>
> Indeed, documentation seems a bit misleading. When they are referencing a plural (bound variables), they are probably referring to a multitude of "single" variables used as keys to a multitude of maps.

I could agree with that, if the sentence didn't start with "The key K
must be...", clearly referencing the single key K in the single map M
in the specific expression "#{ K := V } = M".

> But, documentation aside  that could be corrected; do you think that this kind of implementation is in line with a principle of least astonishment?

Well, I know basically nothing about how maps are implemented, but *I*
would certainly be less astonished by the limitation described in the
documentation (it seems entirely reasonable) than by the actual
limitation, since in most other cases I can think of, you can use "an
expression with bound variables" wherever you can use "a bound
variable".

But maybe it's "just" something that was a challenge to implement, and
will be fixed to match the current documentation in a future release...

--Per

> V/
>
>>
>>> / Raimo
>>>>
>>>>
>>>>
>>>> To make things worse, when I just replace an explicit term, such as {DCID, SORD}, with something like this:
>>>>
>>>>
>>>> f(GRP_KEY), GRP_KEY={DCID, SORD}.
>>>>
>>>> f(V), case MAP of #{ GRP_KEY := V} -> V end.
>>>>
>>>> "some value

>>>>
>>>>
>>>>
>>>> Then the whole thing works again.
>>>>
>>>> I would argue that all cases should just work.
>>>> How come they dont?
>>>>
>>>>
>>>> Kind regards
>>>>
>>>> V/
>>>>
>>
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>

```