Thanks! This is what I had in mind, except that I'm new too Haskell so I tried English instead of code for expressing it :-) I'll also check the IOSpec package Paul suggested, On Mon, Mar 16, 2015 at 7:40 AM, Sumit Sahrawat, Maths & Computing, IIT (BHU) wrote:

On 16 March 2015 at 19:51, Maurizio Vitale wrote:

...

suppose I have a restricted IO monad, RIO that only exposes readFile. and then I have a monad SIO that will eventually provide a virtual file system from a map path->content, also with a readFile function returning SIO(String).

What is the way to write a function parseFile that can operate in both monads so that I can use SIO for testing? should I define a third monad CompileMonad that has instances for both RIO and SIO and then having parseFile :: CompileMonad ast?

You might be able to do something like,

class MonadIO m => ProvidesReadFile m where readFile :: FilePath -> m String

instance ProvidesReadFile RIO where readFile = readFileRIO -- the RIO specific readFile

instance ProvidesReadFile SIO where readFile = readFileSIO -- the SIO specific readFile

parseFile :: ProvidesReadFile m => FilePath -> m ast parseFile = do f <- readFile let ast = parse f -- the pure parser return ast -- works for both monads

...

Thanks,

Maurizio

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This is more suitable for the haskell-cafe. I am posting it there so that more people might comment on it. HTH.

-- Regards

Sumit Sahrawat

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