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Yep, I posted this on r/haskell and someone pointed this out, which is cool. The provided type for Peg didn't derive Enum or Bounded, but it's a good trick to k

Yep, I posted this on r/haskell and someone pointed this out, which is cool. The provided type for Peg didn't derive Enum or Bounded, but it's a good trick to know.

deleted 12y ago
[deleted]
throwaway183839 12y ago
I'm kind of surprised that `enumerate` isn't defined in any standard libraries (as far as I know) since I need it pretty often. After a while spent programming in Haskell you would probably develop your own mini-library of functions that make your life easier. Another one I often find useful is (.:) :: (c -> d) -> (a -> b -> c) -> a -> b -> d (.:) f g = \x y -> f (g x y) which e.g. allows you to define the absolute distance function dist :: (Num a) => a -> a -> a dist = abs .: (-) A fun challenge is figuring out why the definition of (.:) is equivalent to (.:) = fmap . fmap !
- JadeNB 12y ago
  > A fun challenge is figuring out why the definition of (.:) is equivalent to > (.:) = fmap . fmap > ! Of course it's not really equivalent, since the most general type of the LHS is as you have said, whereas that of the RHS involves functor constraints; but inlining the definition of `fmap` for arrows gives the amusing definition: (.:) = (.) . (.) I tend to figure this sort of thing out by successive eta conversion: \f g x y -> f (g x y) \f g x y -> (f . g x) y \f g x -> f . g x \f g x -> ((f .) . g) x \f g -> (f .) . g \f g -> ((f .) .) g \f -> ((f .) .) \f -> (.) (f .) \f -> (.) ((.) f) \f -> ((.) . (.)) f (.) . (.) Is there a better way?
- wz1000 12y ago
  (.:) f g = (f .) . g This can interestingly be generalized(in a way) to get (.::) :: (d -> e) -> (a -> b -> c -> d) -> a -> b -> c -> d (.::) f g = ((f .) .) . g and so on. Since fmap = (.), substitution would get your point free form.
- HanyouHottie 12y ago
  Seems like a better name for it would be (...) :: (c -> d) -> (a -> b -> c) -> a -> b -> d since (...) = (.) . (.) .
- cschneid 12y ago
  The downside of using (.:) is that it is used heavily in other libraries (Aeson for instance) in a very different context (field accessors, similar in look to json's `:`).

Yep, I posted this on r/haskell and someone pointed this out, which is cool. The provided type for Peg didn't derive Enum or Bounded, but it's a good trick to know.

deleted 12y ago

[deleted]

throwaway183839 12y ago

I'm kind of surprised that `enumerate` isn't defined in any standard libraries (as far as I know) since I need it pretty often. After a while spent programming in Haskell you would probably develop your own mini-library of functions that make your life easier. Another one I often find useful is (.:) :: (c -> d) -> (a -> b -> c) -> a -> b -> d (.:) f g = \x y -> f (g x y) which e.g. allows you to define the absolute distance function dist :: (Num a) => a -> a -> a dist = abs .: (-) A fun challenge is figuring out why the definition of (.:) is equivalent to (.:) = fmap . fmap !

JadeNB 12y ago
> A fun challenge is figuring out why the definition of (.:) is equivalent to > (.:) = fmap . fmap > ! Of course it's not really equivalent, since the most general type of the LHS is as you have said, whereas that of the RHS involves functor constraints; but inlining the definition of `fmap` for arrows gives the amusing definition: (.:) = (.) . (.) I tend to figure this sort of thing out by successive eta conversion: \f g x y -> f (g x y) \f g x y -> (f . g x) y \f g x -> f . g x \f g x -> ((f .) . g) x \f g -> (f .) . g \f g -> ((f .) .) g \f -> ((f .) .) \f -> (.) (f .) \f -> (.) ((.) f) \f -> ((.) . (.)) f (.) . (.) Is there a better way?
wz1000 12y ago
(.:) f g = (f .) . g This can interestingly be generalized(in a way) to get (.::) :: (d -> e) -> (a -> b -> c -> d) -> a -> b -> c -> d (.::) f g = ((f .) .) . g and so on. Since fmap = (.), substitution would get your point free form.
HanyouHottie 12y ago
Seems like a better name for it would be (...) :: (c -> d) -> (a -> b -> c) -> a -> b -> d since (...) = (.) . (.) .
cschneid 12y ago
The downside of using (.:) is that it is used heavily in other libraries (Aeson for instance) in a very different context (field accessors, similar in look to json's `:`).