I sum a list of matrices with foldl1 (+)
because I noticed that sum
actually returns the sum of the top left elements as a 1x1 matrix.
$ stack exec --resolver lts-12.5 --package matrix -- ghci
GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help
Prelude> import Data.Matrix
Prelude Data.Matrix> t = [identity 2, identity 2] -- two 2x2 identity matrices
Prelude Data.Matrix> foldl1 (+) t
┌ ┐
│ 2 0 │
│ 0 2 │
└ ┘
Prelude Data.Matrix> sum t
┌ ┐
│ 2 │
└ ┘
This is unexpected for me, LYAH suggests sum = foldl1 (+)
¹ and even hlint suggests I use sum
instead of foldl1 (+)
as is 'removes error on []
' (side question: How do I elegantly and []
-safely sum up matrices?)
Why is sum /= foldl1 (+)
for Matrices, and why isn't generally always sum == foldl1 (+)
?
Or, to exclude the case of the empty list:
Why isn't sum == foldl neutralElement (+)
? or specifically sum == foldl (+) (zero 2 2)
on [identity 2, identity 2]
?
Own work:
Prelude Data.Matrix> :t sum
sum :: (Foldable t, Num a) => t a -> a
Prelude Data.Matrix> :info sum
class Foldable (t :: * -> *) where
...
sum :: Num a => t a -> a
...
-- Defined in ‘Data.Foldable’
Source of sum
is:
sum :: Num a => t a -> a
sum = getSum #. foldMap Sum
Instantiations of Matrix
are:
instance Foldable Matrix where
foldMap f = foldMap f . mvect
instance Num a => Num (Matrix a) where
fromInteger = M 1 1 . V.singleton . fromInteger
negate = fmap negate
abs = fmap abs
signum = fmap signum
-- Addition of matrices.
{-# SPECIALIZE (+) :: Matrix Double -> Matrix Double -> Matrix Double #-}
{-# SPECIALIZE (+) :: Matrix Int -> Matrix Int -> Matrix Int #-}
(M n m v) + (M n' m' v')
-- Checking that sizes match...
| n /= n' || m /= m' = error $ "Addition of " ++ sizeStr n m ++ " and "
++ sizeStr n' m' ++ " matrices."
-- Otherwise, trivial zip.
| otherwise = M n m $ V.zipWith (+) v v'
-- Substraction of matrices.
...
-- Multiplication of matrices.
...
¹ in the context of adding integers
Because the
Data.Matrix
instance forNum
implementsfromInteger
by returning a 1x1 matrix, and+
by addingelementwise
which truncates the right matrix to the size of the left one (or errors if the left is larger than the right).The
sum
isfoldl (+) 0
which starts with a 1x1 matrix from the0
and truncates all of the matrices in the list to that size. Thefoldl1 (+)
does not have to make a matrix from an integer, so the result is the size of the smallest matrix in the list.Also,
sum = getSum #. foldMap Sum
is the defaultFoldable
implementation, but it is overridden by the list instance to besum = foldl (+) 0
.The
Data.Matrix
implementation of+
(as you show above) up to version 0.3.1.1 generates an error if the operands are not the same shape, but in version 0.3.2.0 it assumes they are the same shape without checking, and in version 0.3.3.0 it changed again (according to the comment):The implementation appears to be the same up to the current version 0.3.6.1