Re: [Haskell-cafe] IO (Either a Error) question

On Sat, May 08, 2010 at 07:49:57AM +1000, Ivan Lazar Miljenovic wrote:

Limestraël writes:

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2010/5/1 John Millikin

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You might want to make a local version of ErrorT in your library, to avoid the silly 'Error' class restriction. This is pretty easy; just copy it from the 'transformers' or 'mtl' package.

Yes, I wonder why mtl is not updated so as to remove this restriction.

Presumably because its in "maintenance mode" (i.e. it only gets changed/updated to reflect changes in GHC that might affect it and the API is frozen).

The API isn't frozen -- it can be changed with a library proposal, if you can get people to agree to it. As Ryan said, the Error constraint is there to support a definition of the fail method in the Monad instance for ErrorT. (Personally I think fail is a terrible wart, and should be shunned.)