On 27/03/17 04:49, Waldek Hebisch wrote:
some passage look completely
wrong, like:
+The use of the Vector domain for this does seem to be stretching
+the FriCAS type system a bit. For instance elements of subgroup are
+coded as a vector of a vector (Vector Vector NNI) which would not be
+valid if the type system were fully checked but presumably gets away
+with it because vectors are defined over Type.
Vector(NNI) stores nonnegative integers and provides 1 based
indexing. We do not need most operations provided by Vector,
in particular we do not need vector addition or multiplication
by scalars but that is OK: we use what Vector provides and
nothing more. In other words, we could probably just use arrays
here (but IIRC there was some useful function that only accepted
vectors and did not work on arrays). And using arrays or
vectors to store permutations is a standard trick allowing
resonably fast operations on permutations.
So what have I said that is wrong here?
I know that some languages, like java, have vector types which are
really a type of array. However I suspect that someone trying to
understand an algebra program might expect that when they find a
instance of a Vector domain then it would represent an element of a
vector space. That is it has valid vector addition and scalar
multiplication, they might even think that the type system would enforce
that.
Put yourself in the mind of a potential user or contributor to
permgrps.spad. You come across a structure (Vector) being used in an
unusual way would it not help you to see some explanation that might
warn you of that?
Martin B
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