4 ms·

Can you give an example of the pattern matching in ML and Erlang that is not idiomatic in Haskell?

macintux 7y ago
It's the multiple function heads which (again, I've been informed, no direct knowledge) is discouraged. e.g. https://github.com/macintux/advent-of-code-2019/blob/master/day4/day4.erl#L34-L41 https://github.com/macintux/advent-of-code-2019/blob/master/...
- p0llard 7y ago
  You mean a piecewise definition of a function? I don't think this is unidiomatic or discouraged in Haskell at all.
  - macintux 7y ago
    There's some indirect discussion here that seems to contradict that, but I certainly could be wrong. https://github.com/elm/compiler/issues/973 https://github.com/elm/compiler/issues/973
    - Iceland_jack 7y ago
      That's in the context of Elm, Evan gives his reasons but in the context of Haskell such multi-line equations are very common and normal Haskell style.
- a1369209993 7y ago
  Translating that into Haskell, I get: check_each :: Int -> [Int] -> Bool -> Int -> (Bool,Bool,Int) check_each x (y:_) _ last | y==x && x==last = (True,False,x) check_each x (y:_) _ last | y>x && x==last = (True,True,x) check_each x (y:_) found _ | y>x = (True,found,x) check_each x _ found _ = (False,found,x) is_valid :: (Bool,Bool,Int) -> [Int] -> Bool is_valid (False,_,_) _ = False is_valid (True,True,-1) [_] = False is_valid (True,True,_) [_] = True is_valid (True,found,shortDup) (h:t) = is_valid (check_each h t found shortDup) t which is ugly, certainly, but compiles fine (ghc). The uglyness mainly comes from the data structures, and I don't know enough about the domain to suggest better ones (there might not be better ones).
  - macintux 7y ago
    Well, it was code I threw together quickly for Advent of Code, so try not to be too hard on it.
    - a1369209993 7y ago
      Eh, like I said, that might be the best you can do for a messy problem domain. I'm still not sure what you meant by multiple function heads though.

It's the multiple function heads which (again, I've been informed, no direct knowledge) is discouraged. e.g. https://github.com/macintux/advent-of-code-2019/blob/master/day4/day4.erl#L34-L41 https://github.com/macintux/advent-of-code-2019/blob/master/...

p0llard 7y ago
You mean a piecewise definition of a function? I don't think this is unidiomatic or discouraged in Haskell at all.
- macintux 7y ago
  There's some indirect discussion here that seems to contradict that, but I certainly could be wrong. https://github.com/elm/compiler/issues/973 https://github.com/elm/compiler/issues/973
  - Iceland_jack 7y ago
    That's in the context of Elm, Evan gives his reasons but in the context of Haskell such multi-line equations are very common and normal Haskell style.
a1369209993 7y ago
Translating that into Haskell, I get: check_each :: Int -> [Int] -> Bool -> Int -> (Bool,Bool,Int) check_each x (y:_) _ last | y==x && x==last = (True,False,x) check_each x (y:_) _ last | y>x && x==last = (True,True,x) check_each x (y:_) found _ | y>x = (True,found,x) check_each x _ found _ = (False,found,x) is_valid :: (Bool,Bool,Int) -> [Int] -> Bool is_valid (False,_,_) _ = False is_valid (True,True,-1) [_] = False is_valid (True,True,_) [_] = True is_valid (True,found,shortDup) (h:t) = is_valid (check_each h t found shortDup) t which is ugly, certainly, but compiles fine (ghc). The uglyness mainly comes from the data structures, and I don't know enough about the domain to suggest better ones (there might not be better ones).
- macintux 7y ago
  Well, it was code I threw together quickly for Advent of Code, so try not to be too hard on it.
  - a1369209993 7y ago
    Eh, like I said, that might be the best you can do for a messy problem domain. I'm still not sure what you meant by multiple function heads though.