Convert a Haskell function to SML

Question

I'm trying to convert a Haskell function, which displays a boolean formula, to a SML function. The function: ``` data Formula = Atom String | Neg Formula | Conj Formula Formula | Disj Formula Formula precedence :: Formula -> Int precedence Atom{} = 4 precedence Neg {} = 3 precedence Conj{} = 2 precedence Disj{} = 1 displayPrec :: Int -> Formula -> String displayPrec dCntxt f = bracket unbracketed where dHere = precedence f recurse = displayPrec dHere unbracketed = case f of Atom s -> s Neg p -> "~ " ++ recurse p Conj p q -> recurse p ++ " & " ++ recurse q Disj p q -> recurse p ++ " | " ++ recurse q bracket | dCntxt > dHere = \s -> "(" ++ s ++ ")" | otherwise = id display :: Formula -> String display = displayPrec 0 ``` I' ve come so far as translating it to SML: ``` fun precedence(operator) = case operator of Atom a => 4 | Neg p => 3 | Conj(p,q) => 2 | Disj(p,q) => 1 fun displayPrec dCntxt f = let val dHere = precedence f val recurse = displayPrec dHere val unbracketed = case f of Ato…

Accepted Answer

That would be

```
val bracket =
  if dCntxt > dHere
  then fn s => "(" ^ s ^ ")"
  else fn x => x
```

The function compares the precedence level of your context against the precedence level of the outer operator of your expression and decides to either insert a pair of parentheses around the given string or not.

Convert a Haskell function to SML

Solution

Problem