4 ms·

compare this proposal to the pattern matching you get from Haskell or OCaml and weep. for example (OCaml): let imply = function | (true,false) ->

compare this proposal to the pattern matching you get from Haskell or OCaml and weep. for example (OCaml): let imply = function | (true,false) -> false | _ -> true

macintux 7y ago
Pattern matching across multiple function heads (ML, Erlang) is incredibly liberating. I hear Haskell has it but it's not idiomatic, and I think Elm doesn't support it at all. Frustrating. I've found half-hearted pattern matching solutions like Scala quite disappointing.
- lalaithion 7y ago
  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.
- nv-vn 7y ago
  OCaml, F#, ReasonML don't support that, I think it's unique to SML in the ML family.
  - macintux 7y ago
    Interesting, thanks. SML is my only direct exposure to the family, and I recall a conference speaker mentioning that ML (perhaps SML) programmers weren't fond of the feature.

compare this proposal to the pattern matching you get from Haskell or OCaml and weep. for example (OCaml): let imply = function | (true,false) -> false | _ -> true

macintux 7y ago

Pattern matching across multiple function heads (ML, Erlang) is incredibly liberating. I hear Haskell has it but it's not idiomatic, and I think Elm doesn't support it at all. Frustrating. I've found half-hearted pattern matching solutions like Scala quite disappointing.

lalaithion 7y ago
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.
nv-vn 7y ago
OCaml, F#, ReasonML don't support that, I think it's unique to SML in the ML family.
- macintux 7y ago
  Interesting, thanks. SML is my only direct exposure to the family, and I recall a conference speaker mentioning that ML (perhaps SML) programmers weren't fond of the feature.