KeyValuePattern in matching and replacement in nested associations

Question

This case revealed I am missing something in pattern matching and evaluation in case of match, or I can't grasp it due to the busy week I have. Either way, I find this problem interesting for wider audience:

asso = <|
    "key" -> <|"a" -> "1", "b" -> Compress@"test"|>
|>

We want to replace the inner association with uncompressed value from "b" key. The expected result is:

<|"key" -> "test"|>

Association is HoldAllComplete so we will have to use Trott-Strzebonski technique or RuleCondition.

The problem is it works but throws a message in between:

asso /. KeyValuePattern[ {"a" -> "1", "b" -> val_String}] :> With[
    {eval = Uncompress[val]}
  , eval /; True
]
 (*or RuleCondition@Uncompress[val]*)

Uncompress::argt: Uncompress called with 0 arguments; 1 or 2 arguments are expected.
<|"key" -> "test"|>

The question is, what am I missing? Recent chat discussion confirms that is not obvious.

I have stuff to do so I'm just using the workaround:

asso /. sub : KeyValuePattern[{"a" -> "1", "b" -> _String}
] :>   RuleCondition @ Uncompress @ sub @ "b"

but I don't get the difference that makes it work.

Answer 1

The output of this code in Open Cloud seems to reveal what is happening:

<|"a" -> 1, "b" -> 2, "c" -> 3, "d" -> 4|> /.
  KeyValuePattern[{"a"->v1_, "b"->v2_, "c"->v3_, "d"->v4_, "x"->v5_}] :> 
    "foo" /; (Print[{v1},{v2},{v3},{v4},{v5}];True)

{1}{}{}{}{}

{1}{2}{}{}{}

{1}{2}{3}{}{}

{1}{2}{3}{4}{}

<|a->1,b->2,c->3,d->4|>

So we can see that the expression is incrementally tested for partial matches.

Rather like:

{1, 2, 3, 4} /. {x__, ___} :> "foo" /; Print[{x}]

{1}

{1,2}

{1,2,3}

{1,2,3,4}

{1, 2, 3, 4}

This is how we get Uncompress[] in your evaluation as val_String is given a null (vanishing) match in the first attempt.