Code in F#.

Implement the function desugarChoose : expr -> expr, which should convert the given expression to an equivalent expression that does not use Choose. You will need to use Try and Fail. Be careful to ensure that you desugar all subexpressions.

let rec desugarChoose (e : expr) : expr =

failwith "Not implemented"

Here are the test cases to try:

// > eval (desugarChoose (Choose [Fail; Fail; Fail; Num 0])) [];;

// val it: value = VNum 0

// > eval (desugarChoose (Choose [Fail; Fail; Fail; Fail])) [];;

// val it: value = VFail

// > eval (desugarChoose (Choose [Choose [Fail; Num 0]; Choose [Fail]])) [];;

// val it: value = VNum 0

// > eval (desugarChoose (Choose [Choose [Fail]; Choose [Fail; Num 0]; Choose [Fail]])) [];;

// val it: value = VNum 0

// > eval (desugarChoose (Choose [ Fail; Or (Choose [ True ; Var "x" ], Num 0)])) [];;

// val it: value = VBool true

let e2 =

desugarChoose (Plus (Var "y",

Choose

[ If (LessThan (Var "x", Num 0), Fail, Var "x")

; If (LessThan (Plus (Var "x", Var "y"), Num 0), Fail, Plus (Var "x", Var "y"))

; Choose [ Var "y" ]]))

// > eval e2 ["x", 10; "y", 20]

// val it: value = VNum 30

// > eval e2 ["x", -15; "y", 20]

// val it: value = VNum 25

// > eval e2 ["x", -15; "y", -20]

// val it: value = VNum -40