Re: [Haskell-cafe] Errors in non-monadic code

Yeah, non-monadic is not the best term... The problem is that it's always so hard to communicate when you want to say a total function that is not in the context of the IO monad. There should be a simple, short name for these functions, so we can easily talk about them. What ends up happening a lot of the times is that people on the mailing list call these "pure" functions, and immediately they get an email saying that Haskell is pure, so everything is a pure function. In the end, it doesn't really help... Jose On Mon, Aug 19, 2013 at 10:09:13PM +0400, Daniel F wrote:

On Mon, Aug 19, 2013 at 9:48 PM, wrote:

...

Hi,

Hello!

...

What is the proper way to implement a non-monadic function that checks whether a given value is correct and gives a proper error message otherwise ? What is the recommended option ?

I am not sure, what do you mean by non-monadic. Both (Either String) and Maybe are monads.

You can pick up whatever option you like, depending on which option, in your opinion, suits you better for your specific case. There is also a helpful errors [1] package that provide convenient means of converting between the results of the two approaches.

Control.Error.Util.hush :: Either a b -> Maybe b Control.Error.Util.note :: a -> Maybe b -> Either a b

...

* Either String a

check val | valid val = Right val | otherwise = Left errorMsg

* Maybe String

check val | valid val = Nothing | otherwise = Just errorMsg

Cheers, Jose

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[1] http://hackage.haskell.org/package/errors

-- Sincerely yours, -- Daniil