Using Haskell to interpret multi-line strings of code at runtime

Question

Problem:

I would like to be able to interpret any sort of Haskell code at runtime - also code that is not bound by a single line.

Right now I am using hint to do this below, which works with single lines:

html :: String -> IO String
html code = do    
    r <- runInterpreter $ do
        setImports ["Prelude"]
        interpret code (as :: () -> String)

    case r of
        Left err -> return $ show err
        Right func -> return $ func()

Example:

If code from above is \() -> "Hello World" that works.

But if code is something like this below, my code from above does not work: (Update: it does).

\() -> let concatString :: String -> String -> String
           concatString str1 str2 = str1 ++ str2
       in concatString "Hello" "World"

Question:

How do I interpret multi-line Haskell strings at runtime using hint or any other library?

---

Update

This does work - I made a mistake in the original expression (which I have now corrected).

Answer 1

I haven't used hint, but I can tell you that your example is not a valid Haskell expression. where clauses are not attached to expressions, they are attached to definitions. That is, you have to have an = sign to be able to have a where clause.

-- Correct
foo = bar
    where
    bar = baz
        where 
        baz = 42

-- Incorrect
foo = (bar + 1 where bar = 41)

If you want to define something in expression context you must use let

let concatString :: String -> String -> String
    concatString str1 str2 = str1 ++ str2
in concatString "Hello" "World"