[erlang-questions] Illegal map key in pattern

[Valentin Micic]

Thank you, I guess…


> On 05 Aug 2019, at 09:29, Raimo Niskanen <raimo+erlang-questions@REDACTED> 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
> dark on your gripe.
> 
>    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.
> It's a known limitation.
> 
> / 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/
>> 
> 
> 
> -- 
> 
> / Raimo Niskanen, Erlang/OTP, Ericsson AB
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