Better handling of Hash with default block but of the wrong type

[Blacksmoke16]

Discussion

• What aspect of the language would you like to see improved?

I'd like to see the error message related to missing hash keys improved to make some edge cases more clear.

• What are the reasons?

I kept getting a Missing hash key exception and was very confused because the exception included the key and because of the default block, should not be possible. It ultimately was because of

crystal/src/hash.cr

Line 1193 in 46ca8fb

if (block = @block) && key.is_a?(K)

where my key wasn't actually the type I thought it was raising the error vs calling the block.

• Include practical examples to illustrate your points.

record SomeThing, message : String do
  def inspect(io)
    @message.inspect io
  end
end

hits = Hash(String, Hash(Int32, Int32)).new { |hash, key| hash[key] = Hash(Int32, Int32).new(0) }

hits["foo"][0] = 2 # Fine

thing = SomeThing.new "bar"

# Very confusing because it _looks_ like a string.
hits[thing][0] = 3 # Unhandled exception: Missing hash key: "bar" (KeyError)

• Optionally add one (or more) proposals to improve the current situation.

Solutions I could think of are either to maybe have a dedicated error path when there is a block but its type doesn't match, or include the type of the key in the existing error message.

[devnote-dev]

I do wonder why Hash#[] and Hash#[]? aren't strictly typed, that solves this with a compile time error. I guess it's too late to change it now...

[Blacksmoke16]

@devnote-dev See #8893.

[straight-shoota]

I suppose it should be trivial to enhance the error message when @block && !key.is_a?(K) to include something like (key is not #{ K }).