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).

luqui · Accepted Answer · 2019-01-12 01:42:29Z

3

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"

answered Jan 12, 2019 at 1:42

luqui

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2 Comments

jundl77 Over a year ago

You're right! And that was also the issue, using the let statement works, also with multi-lines. Amazing! Is there no way to attach definitions to expressions in Haskell?

luqui Over a year ago

There is, and it's let. let : expression :: where : definition. You can even put a where clause after a definition you make in let (I think)

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