[Haskell-cafe] ClassyPrelude vs. Haskell.Language.Interpreter

Corentin Dupont corentin.dupont at gmail.com
Tue May 26 22:11:04 UTC 2015

Lazy text is this one?

At first sight I'd say it's the way lazy text works with reverse that the
interpreter doesn't like...

On Tue, May 26, 2015 at 7:47 PM, Mike Meyer <mwm at mired.org> wrote:

> No answer on -beginners, so I'm trying -cafe.
> I'm trying to run interpreted code via ClassyPrelude, and getting some
> results that make me suspect a bug in the Prelude's type system. Or maybe
> the interpreter.
> Anyway, here's a bit of code that works as expected:
> {-# LANGUAGE NoImplicitPrelude #-}
> import ClassyPrelude
> import Language.Haskell.Interpreter
> main :: IO ()
> main = do
>   fun <- runInterpreter $ makeFun "reverse"
>   case fun of
>    Left e -> print e
>    Right f -> readFile "/etc/motd" >>= hPut stdout . f
> makeFun expr = do
>   set [languageExtensions := [NoImplicitPrelude]]
>   setImportsQ [("ClassyPrelude", Nothing)]
>   interpret expr (as :: Text -> Text)
> I don't think I can simplify this any further. It works as expected, and
> also works as expected, and prints out the contents of /etc/motd reversed.
> However, if you change the type signature in the last line from Text ->
> Text to LText -> Ltext (to get lazy text), you get no output. But if you
> change the function in the first line after main from "reverse" to "id", it
> works.
> So far, it might be an issue with lazy IO. However, change the type
> signature in the last line to LText -> Text. In this case, there is no
> output for either value of the expression.  I expect an error in this case,
> as neither id nor reverse should be able to have the type LText -> Text!
> So, is there something I missed in either ClassyPrelude or the
> Interpreter? Or is this a subtle interaction, in which case can someone
> suggest a workaround? Or have I found a bug in one of the two?
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