Computed variables in violation messages

[tpluscode]

I was just looking at #204 and its discussion and I had this idea for binding computed values for interpolating in the message.

For example, @VladimirAlexiev writes

sh:minCount, sh:maxCount should bind ?value to the actual number of occurrences found

If that is confusing (because ?value is supposed to be an actual resource found in the graph, not a count), then maybe a new variable ?_count should be introduced.
I used an underscore because the name ?count may well conflict with a variable that the shape author wanted to use for a different purpose

To bind the actual count to a variable, we could use a Node Expression

[
  sh:path sh:name ;
  sh:maxCount 1 ;
  sh:message "Expected {$maxCount} name but found {$count}." ;
  sh:messageVariable [
    sh:name "count" ;
    sh:value [
      sh:count [
        sh:nodes [ sh:path sh:name ] ;
      ] ;
    ] ; 
  ] ;
] .

Note 1: the usage of sh:value violates its rdfs:domain
Note 2: sh:nodes [ sh:path sh:name ] ; unnecessarily duplicates the Property Path. We could introduce a special Node Expression to abbreviate this to sh:nodes [ sh:activePath () ] similarly to sh:activeNode suggested by @HolgerKnublauch

Going one step further, I would even allow Node Expression to completely replace literal sh:message

[
  sh:path sh:name ;
  sh:maxCount 1 ;
  sh:message [
    sparql:concat (
      "Expected"
      [ sh:var "maxCount" ] 
      [ 
        sh:if [ sparql:eq ( [ sh:var "maxCount" ] 1 ) ] ; 
        sh:then "name"; 
        sh:else "names" 
      ] ;
      "but found"
      [ sh:var "count" ] 
    ) ;
  ] ;
  sh:messageVariable [
    sh:name "count" ;
    sh:value [
      sh:count [
        sh:nodes [ sh:path sh:name ] ;
      ] ;
    ] ; 
  ] ;
] .

Note 3: how would that handle multiple languages though?

What do you think?

[tpluscode]

@HolgerKnublauch funny how I was mildly skeptical about binding variables and yet here I am, coming up with a potential usage already 🙈

[HolgerKnublauch]

Yes, excellent idea. Node Expressions at sh:message can solve this and we could introduce something like sh:printf as a node expression type to generate such strings easily.