Le 28 févr. 07 à 02:57, skaller a écrit :

> After all a nominal type is just a structural type with
> a unique tag representing its unique name.

What you miss here is the _process_ behind nominal types.

(paraphrasing Jacques) Types are not precise enough to define the  
semantic of a function, multiplication and addition have the same  
type but in many cases you cannot exchange one for the other. With  
that respect structural typing is a fallacy, it is not because types  
match that you can replace a function by another.

With a nominal type the programmer signals explicitely its intent to  
support a semantic. Notably you cannot use a function in a given  
context if it was not designed for that purpose which sounds  
fascistic but makes sense from a software development point of view.

Structural typing would make sense if it involved the weakest  
invariants a function. But for now, because of the poor information  
types give about semantics, structural typing is just bureaucratic  
compliance whereas nominal typing is a semantic contract --- whether  
actually fulfilled or not is another (undecidable) question.

Daniel