21 Dec
2007
21 Dec
'07
1:41 p.m.
On Dec 21, 2007 12:08 PM, Nicholls, Mark
I thought from
"Num numberType => SquareConstructor numberType"
We could deduce that (in English rather than get Haskell and FOL confusion)
all values of "SquareConstructor a"….the type of a would have be be in class Num?..
is this not correct?....if not….why not?
That's a reasonable thing to assume. It just happens that Haskell doesn't
work that way. There's an asymmetry between constructing and
pattern-matching, and it's one that many people have complained about.
Personally, I never use class contexts in data declarations, simply because
it's too easy to get confused about what they do and do not guarantee.
--
Dave Menendez