[Haskell-cafe] Arithmetic expressions with random operators

[Francesco Ariis]

On Thu, Jun 08, 2017 at 04:03:55PM +0300, Mahmoud Murad wrote:
> I have the following problem, I want to randomly generate arithmetic
> expressions based on a number, for example:
> if I have n = 3, then the expression must have 3 operators like this
> (4)*(((3+5)-2)).

Hello Mahmoud,
    probably not the most flexible and posh solution, but datatypes
would do.

    data Exp = EInt Integer
             | Sum  Exp Exp
             | Diff Exp Exp
             | Mult Exp Exp

and then of course you want to write

    evaluate :: Exp -> Integer

and

    write :: Exp -> String
    -- maybe instance Show Exp where

Once you do that, picking Exp constructors at random should not be
difficult.

There are other ways to write a DSL like this (using GADTs, tagless
final style, etc.), each of those expands on/solves a specific problem
(extensibility, etc.); but there is no benefit in complicating your code
if you don't need it.

Does that help?
-F