It is said, next ghc is going to support the included overlapping
instances.
At this point, i have a question to the Haskell experts and
implementors, concerning the duplicate definitions.
For example, program the determinant of a matrix:
---------------------------------------------------------------------
class Num a => Field a where divide :: a -> a -> a
data Matrix a = Mt [[a]]
det :: Num a => Matrix a -> a
det (Mt rs) = expandByRow rs
where
expandByRow rs = ...something with +,-,*
det :: Field a => Matrix a -> a
det (Mt rs) = gaussMethod rs
where
gaussMethod rs = ...something with +,-,*,divide
----------------------------------------------------------------------
For the case of det m, m :: Field a => Matrix a,
the compiler has to set the second definition of det. For under this
stronger condition, the better algorithm is possible - gaussMethod.
Otherwise, the compiler has to choose the first definition - at least,
it yields the correct result.
At this, Haskell reports an error: duplicated definitions for det.
Providing the two functions, say, det, det_f for the user to choose
is an ugly way out: the mathematical map is the same, why call it
different? In other words, the values in the language are not
polymorphic enough.
So, try to help this by using ghc with -fallow-overlapping-instances
and by making `det' a class method:
----------------------------------------------------------------------
class WithDet a where det :: Matrix a -> a
instance Num a => WithDet a where det (Mt rs) = expandByRow ...
instance Field a => WithDet a where ... gaussMethod ...
----------------------------------------------------------------------
will ghc -c -fglasgow-exts -fallow-overlapping-instances
allow this?
If it will, then could the compiler resolve in the "similar" way the
duplicated det definitions in the first program - and thus provide
the corresponding language extension?
------------------
Sergey Mechveliani
[EMAIL PROTECTED]
http://www.botik.ru/~mechvel
