Question

I’m struggling with converting a Right String. I have a method called getEval which evaluates a math expression (a String basically), and I get it back as follows:

*Main> getEval "3+6"
Right "9"

the type of getEval is

*Main> :t getEval "3+6"
getEval "3+6"
:: (Functor m, Control.Monad.CatchIO.MonadCatchIO m) =>
   m (Either InterpreterError String)

getEval looks like this:

getEval str = runInterpreter $ setImports ["Prelude"] >> eval str

If it matters, getEval uses the eval in Haskell’s hint library (Language.Haskell.Interpreter) to do the actual parsing and calculation.
How can I get this into a float?

Answer 1

getEval str = do
        Right res <-  Interpreter.runInterpreter (Interpreter.setImports
                ["Prelude"] >> Interpreter.interpret str (Interpreter.as ::
                Float))
        return res