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Musings on the tagged language (part 3) (2015 February 15)

In a previous entry (see 2015 February 14 bis) I discussed an idea to solve the problem of having Lispy syntax in the tagged language, without the burden of exporting and importing from libraries a big number of identifiers bound to class methods and field accessors.

The possible solution

In the expression:

(length "ciao")

the literal expression "ciao" is considered of type <string> and the symbol length, that comes before it, is searched in the list of multimethods whose first argument is of type <string>. The class <string> contains a table of multimethods whose first argument is of type <string>. There is no need to import a binding named length to access the table of multimethods: if it is determined at expand–time that an expression returns a value of type <string>, the expander has access to the table.

But …

What is the difference between having a table of multimethods in a class and just having a global namespace of multimethod names that is queried before the bindings in the local lexical context? In the program:

(import (vicare))

(length "ciao")

the expander just visits the global namespace first, in search of a method named length having one argument with type <string>:

A global namespace of multimethods is equivalent to multimethods in the class definition.

If it is a global namespace …

Is it fine to access it transparently? Or would this transparent access make the code too difficult to interpret for humans? Should a (possibly optional) namespace qualifier be introduced? Possible alternatives:

About the improper list syntax

It is currently unused by Vicare. The Scheme reader could be instructed (in #!vicare mode) to convert the input:

(append . "ciao" " mamma")

to some special form, like it already does for the apostrophe and similar operators: the input '(1 2 3) is read as (quote (1 2 3)). So, in general:

(?form0 ?form1 … . ?form2 ?form3 …)

could be read as:

(improper (?form0 ?form1 …) (?form2 ?form3 …))

where improper must be an identifier bound in the lexical context of the expression.

But what about:

'(1 2 . 3)

which is a perfectly valid literal expression? The reader should distinguish between quoted and unquoted input: this should not be a reader’s responsibility.

Conclusions

Having a global namespace for multimethod names seems the most sensible solution.

The cleanest way to query this namespace seems to be a proper syntax use like:

(method-call ?name ?operand ...)

where method-call is defined in the boot image and exported by (vicare). Some reader syntax should be used to obtain a method-call syntax use from some easy–to–type sequence of characters.


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