F# Discriminated Unions and Printing

Question

I am trying to solve the problem below, but have difficulties figuring out how to print it in the desired format, with parenthesis and numbers / operators.

I have the code below to create expressions, but printing %A using match pattern does not work. I can access values but cannot print them in the desired format. Anyone has suggestions?

let one = Const(1)
let two = Const(2)
let three = Const(3)
let Bin1 = BinOpr(one, "+", two)
let Bin2 = BinOpr(Bin1, "*", three)

Answer 1

The solution to this problem is to implement toString by pattern matching on expr and output the appropriate string for Const and BinOpr :

• For Const , you just need to convert your int to a string.
• For BinOpr , you have to build a string of the form (<expr> <op> <expr>) .

Try to implement the function yourself, it's not that difficult but in the case you get stuck, I provide the solution below.

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Solution

let rec toString expr =
  match expr with
  | Const x -> string x
  | BinOpr (e1, op, e2) -> sprintf "(%s %s %s)" (toString e1) op (toString e2)