@hackage compose-trans0.1

Composable monad transformers

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    • 0.1 Sat, 5 Nov 2011
    • 0.0 Mon, 17 Aug 2009

A version of monad transformers that (a) allows one to convince the type checker that transformer application is a monad, and (b) doesn't need lots of boilerplate to add a new transformer. It's supposed to play nicely with Control.Monad.Trans.

In order to make a new transformer (say, T) an instance of TransM (TransP, TransF) class, all you have to do is:

  • define instance MonadTrans T — which you've probably done already

  • define instance Monad m => Monad (T m) — also something quite common (for TransP and TransF you'd need another instance for MonadPlus and MonadFix, respectively).

  • write instance Trans T where transMInst = instM — which is exactly one line of boilerplate. Sorry about that.

After that, you can use your new and shiny transformer in compositions, like ReaderT Char :. T :. WriterT String — and such a composition would automagically become a monad transformer.