Question

I’m having some trouble trying to incorporate a read and write statement into evaluateStatementInner function.

I’ve tried changing the return types of both functions without success.

I’m trying to convert a type IO(Env) into Env. I know that this can be done with binding in a function that returns an IO action but can I do it in the evaluateInner statement?

Cheers

Answer 1

Evaluating Read and Write by necessity involves performing i/o. So bite the bullet and change the type of evaluateStatementInner to Stmt -> Env -> IO Env (or, better, incorporate it directly into evaluateStatement).

You will then need to modify evaluateListOfStatements to be of type [Stmt] -> Env -> IO Env.

---

So this now gives you a compile error

case stmt of
...
    While boolExp innerStmt ->
        if evaluateBoolExp boolExp env
            then evaluateStatementInner (While boolExp innerStmt) (evaluateStatementInner innerStmt env)
            else env

because evaluateStatementInner innerStmt env yields an IO Env, but evaluateStatementInner (While boolExp innerStmt) wants an Env.

This is a case for monadic bind (and note the else branch needs updating to have the correct type as well):

case stmt of
...
    While boolExp innerStmt ->
        if evaluateBoolExp boolExp env
            then evaluateStatementInner (While boolExp innerStmt) =<< evaluateStatementInner innerStmt env
            else return env

You might not be comfortable with the monadic operators, so I’ll translate it into do-notation:

case stmt of
...
    While boolExp innerStmt ->
        if evaluateBoolExp boolExp env
            then do env' <- evaluateStatementInner innerStmt env
                    evaluateStatementInner (While boolExp innerStmt) env'
            else return env