[erlang-questions] Illegal map key in pattern

[Per Hedeland]

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 doesnt:
>>
>>
>>   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
> dark on your gripe.

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

> / 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 dont?
>>
>>
>> Kind regards
>>
>> V/
>>
> 
>