[nem-en] recursive types

[Kamil Skalski]

On Thu, Sep 23, 2004 at 03:47:36PM +0200, Wiktor Zychla wrote:
> yet another question on pattern matching.
> 
> how do I tell the compiler that one of branches should have no code (return 
> void)?
> 
> in following scenario:
> 
> public Foo( l : Lambda ) : void {
>    match ( l ) {
>        | Abs( ... ) => ...
>        | App( ... ) => ...
>        | _ => ????
>    }
> }
> 
> dropping the "_" branch at all causes the warning "matching 
> non-exhaustive". to eliminate the warning I guess I need to put something 
> there.
> 
> since I expect the match body to return void, I need to put something in 
> place of ????.
> 
> putting the the junk there helps:
> 
> public Foo( l : Lambda ) : void {
>    match ( l ) {
>        | Abs( ... ) => ...
>        | App( ... ) => ...
>        | _ => mutable i = 0; i=1;
>    }
> }
> 
> but this in not a solution. could you provide a more elegant one?
> 
> interestingly, dropping the "=> body" causes somewhat misleading error:
> 
> public Foo( l : Lambda ) : void {
>    match ( l ) {
>        | Abs( ... ) => ...
>        | App( ... ) => ...
>        | _
>    }
> }
> 
> "found junk after pattern"
> 
> ;)
> 
> actually the expected message would be rather "did not find anything after 
> pattern".

:) Probably we should make it more user-friendly message.

Kamil