Messages with Association arguments

[maxitg]

Changes

• Usually, messages in Wolfram Language take numbered arguments.
• However, Failure objects have an Association as a second argument, which fits perfectly with specifying the arguments of the failure with descriptive keys.
• Then, it would be convenient to be able to turn that Failure object directly into a message which would take the same keys as arguments.
• The built-in Message function, however, does not support passing Association arguments to the message strings.
• This PR implements PackageScope declareMessage and message that allow one to do that.
• message can also directly take Failure objects and create messages from them.

Comments

• This will be particularly important for the Multihistory system where conversion functions and properties will need to report errors "far away" from where the messages will be produced.
• The idea is that these functions will throw Failure objects which will then be caught by a function from the Multihistory infrastructure which will then generate a message with an appropriate head depending on which symbol triggered the conversion/property computation.

Examples

• Declare a message:

  declareMessage[General::vertexNotFound, "Could not find vertex `vertex` in hypergraph `hypergraph`."];

• Generate a message:

  message[hypergraphNeighborhood::vertexNotFound, <|"hypergraph" -> {{1, 2, 3}, {3, 4, 5}}, "vertex" -> 0|>];

• You can also create a message directly from a Failure:

• All this technology allows for a very simple implementation of message-throwing functions:

expr : HalfInteger[x_] := ModuleScope[
  result = Catch[halfInteger[x], _ ? FailureQ, message[HalfInteger, #, <|"expr" -> HoldForm[expr]|>] &];
  result /; !FailureQ[result]
];

halfInteger[x_ ? EvenQ] := x / 2;

declareMessage[HalfInteger::odd, "The number `number` in `expr` is odd."];
halfInteger[x_ ? OddQ] := throw[Failure["odd", <|"number" -> x|>]];

In[] := HalfInteger[4]
Out[] = 2

In[] := HalfInteger[3]
During evaluation of In[] := HalfInteger::odd: The number 3 in HalfInteger[3] is odd.
Out[] = HalfInteger[3]

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This change is 

[maxitg]

@taliesinb, @daneelsan, this is ready for review. I don't plan to implement anything else here.

[maxitg]

@taliesinb, @daneelsan, could you give an ETA on when can you look at this (I need it for Multihistory)?

[daneelsan]

@maxitg Looking at it now

[daneelsan]

It seems there is/was an attempt to make Message work with templates:

In[] := Message[MessageObject[<|"MessageSymbol" -> HoldForm[b], 
    "MessageTemplate" -> "Hello `world`!", 
    "MessageParameters" -> <|"world" -> 3|>|>]]; $Failed
Hello 3!

Out[] = $Failed