[Haskell-cafe] SML vs Haskell types

Albert Y. C. Lai trebla at vex.net
Mon Mar 15 23:48:00 UTC 2021

You're now looking at the difference between:

-- list of booleans but the end has an Int
data T1 = End1 Int | Node1 Bool T1
whatInt1 (End1 i) = i
whatInt1 (Node1 _ xs) = whatInt1 xs


-- list of booleans but the end has an Int again but different
data T2 = End2 Int | FalseNode2 T2 | TrueNode2 T2
whatInt2 (End2 i) = i
-- the following two cases are unmergeable
-- but there is a mitigation if you find out about "record syntax"
whatInt2 (FalseNode2 xs) = whatInt2 xs
whatInt2 (TrueNode2 xs) = whatInt2 xs

Record syntax can mitigate it:

data T3 = End3 Int | FalseNode3 {tail3 :: T3} | TrueNode3 {tail3 :: T3}
whatInt3 (End3 i) = i
whatInt3 r = whatInt3 (tail3 r)

But I say "mitgate", not "solve", because it doesn't generalize to

data Q = QI Int
        | QF {qtail :: Q} | QT {qtail :: Q}
        | BF{qleft, qright :: Q} | BT{qleft, qright :: Q}

On 2021-03-12 7:59 p.m., Galaxy Being wrote:
> However, I'm still wondering how to have an abstracted (x:xs) - like 
> pattern to collapse all the ingredients, i.e.,
> whatHolder2 (Holder2 (sh)) = sh
> whatHolder2 (shish2-head (shish2-tail)) = whatHolder2 shish2-tail
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