Nicolas Pouillard wrote:
Excerpts from Neil Brown's message of Tue Nov 03 13:45:42 +0100 2009:
Hi,
I was thinking about some of my code today, and I realised that where I
have an arrow in my code, A b c, the type (A b) is also a functor. The
definition is (see
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Arrow.html):
fmap = (^<<)
-- Or, in long form:
fmap f x = arr f <<< x
Out of curiosity, and since this is a typical haskell-cafe question,
does this definition of fmap hold for all arrows?
Yes, as shown by the 'WrappedArrow' newtype:
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Applicative.html#t%3AWrappedMonad
While I don't wish to suggest that "all arrows are functors" is false, I
think the argument "yes, because this library says so" is not too
strong. Let us not forget, according to *the libraries*, Double is in
Enum - which, I think you'll agree, is just weird...
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