On 11/08/2012, at 9:38 AM, john skaller wrote:
>
> Somehow this breaks my test program (I get an overload error in C++
> for an assignment).
This is due to the following design fault:
(1,(2.2,"x"))
Unfortunately the current matching rules match
U ** V with U = 1, V = 2.2,"x"
and then V matches A ** B with A = 2.2, B ="x"
making it look like 1,2.2,"x"
Two solutions come to mind. The first is to insist the tail V is at least
2 elements long.So U ** V will not match X * Y, only X * Y * Z.
This stops the recursion before Felix gets confused between a tail
of one element which is not a tuple and one which is. The confusion
arises because the proper rule is that the tail MUST be of form X ** Y
itself.
The second solution seems cleaner: may U ** V match ANYTHING.
If it is a nontuple T, set U = T and V = terminal. If its terminal, U and
V are both terminal. Terminal might be either unit or void. Unit seems
wrong: although
1, ()
is a valid tuple, Felix like to get rid of units using X * 1 = X
(i.e. applying the isomorphism). void avoids this problem,
but there are no values of type void to return. I'm not sure
matching against
X ** void
to detect the end of the "list" will work. However it seems right that
X ** void = X * unit (up to isomorphism)
Since a type variable cannot be void we would need two matches:
C[X ** void]
C[X ** Y]
where the first is most specialised and prevents Y being void.
The "unit" solution leads to an infinite list:
X * Y = X ** Y ** 1 ** 1 ** 1 ** 1 ....
which of course can be trapped the same way as above.
--
john skaller
skal...@users.sourceforge.net
http://felix-lang.org
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