[Haskell-cafe] Simple matrix

[Chris Kuklewicz]

Mathematically the least surprising thing for matrices/arrays is

(fromInteger 0) * a{- n by m Matrix -} = "0"{- n by m Matrix -}
(fromInteger 1) * a{- n by m Matrix -} = a{- n by m Matrix -}

Thus I would want (fromInteger 1) in this case to make an Identity {- n by n
Matrix -} matrix.  And then (fromInteger i) to be a diagonal n by n matrix of
all i's.

There is no reason to have infinite columns or rows from (fromInteger i), it
would only produce square diagonal matrices with i on the diagonal.

This has the very nice property that Num ops "lift" from integer to matrices:

For type Matrix: 2+3 == 5, 2*3 == 6,  2*(6-3) == (2*6)-(2*3) == (negate 6), etc.
and (negate (fromInteger 4)) == (fromInteger (negate 4))

Mathematically, I can't remember ever wanting to add x to every entry in a
matrix.   Remeber: (+) and (*) have type "Matrix -> Matrix -> Matrix".  If you
want to add Int to Matrix then you should really define a new operator for that.

Note: For a purist Num should be commutative, which means only square Matrices
are allowed.

If you must use (*) and (+) in bizarre ways, then you could hide the Prelude and
  substitute your own Math type classes that know how to mix your types.

Atila Romero wrote:
> Although there *could* be a fromInteger default behavior, there isn't a 
> mathematical default behavior to c+A.
> An even c*A it's hard to make work, because an identity matrix only 
> works if it is a square matrix.
> Example, if in c*A we make
> A=
> 1 3
> 2 4
> and
> c=
> c 0 0 0 ...
> 0 c 0 0 ...
> 0 0 c 0 ...
> 0 0 0 c ...
> ...
> the result will have 2 lines and infinite columns. And if we make A*c 
> the result will have 2 columns and infinite lines.
> And since there's no way to tell to fromInteger which size we need for 
> c, there's no way to make fromInteger works in a intuitive way.
> 
> So, I think it's better to just not use fromInteger at all, because it 
> will work at some cases but will give wrong results at others.
> 
> Atila
> 
> Bjorn Lisper wrote:
>> Udo Stenzel:
>>  
>>> Bjorn Lisper wrote:
>>>    
>>>> - your definition of fromInteger will behave strangely with the 
>>>> elementwise
>>>>   extended operations, like (+). 1 + [[1,2],[3,4]] will become
>>>>   [[2,2],[3,5]] rather than [[2,3],[4,5]]. Array languages 
>>>> supporting this
>>>>   kind of overloading invariably have the second form of semantics.
>>>>       
>>> Don't call an array a matrix.  If is named matrix, it should have
>>> matrix multiplication, addition, and they should obey the expected 
>>> laws.      
>>
>> But you still have the problem with the overloading of constants in your
>> proposal. If you write 17 + a, where a is a matrix, what do people in
>> general expect it to be?
>>
>> Björn Lispe