[erlang-questions] Illegal map key in pattern

[Raimo Niskanen]

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