On 22/06/2012, at 12:14 AM, john skaller wrote:
>
> Interestingly there's no problem with ^, the problem is with *.
> Felix can handle ^ because it's a left associative binary operator
> so any number of these can be handled by recursion (Felix type
> functions can be recursive). But operator * is not binary but
> n-ary.
Aha! I know how to fix this now!
A * B * C -> flatten ( a \* b \* c)
where \* is left associative. The important thing: Felix already
has a flatten function, but it doesn't work. The reason is flatten
above isn't a function, its a **constructor** just lie * \* etc.
If this is to work then src/lib/std/tuple.flx should reduce to a single
set of binary cases.
Roughly:
instance[T,U] Str[T*U] {
fun str (t:T, u:U) => "("+str t + ", " + str u+")";
}
becomes recursive, matching first T, but U is "rest of the tuple".
So we have to write instead:
instance [T,U] Str[ flatten (T \* U)] {
fun str (t:T, u:U) => "("+str t + ", " + Str[U]::str u+")";
}
Note that here the str u is now matching the "rest of the tuple"
which has 1 less component. This calls str recursively,
but note this is NOT ordinary recursion, it's polymorphic
recursion. We also have to code a "final" case to stop
the recursion. Normally this would be unit, but that messes
things up so we will terminate with a final binary T * U
(not flattern T \*U). Hmm ....
--
john skaller
skal...@users.sourceforge.net
http://felix-lang.org
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