Henning Thielemann wrote:
On Sun, 9 Sep 2007, Stuart Cook wrote:
When combining monadic and non-monadic code, I've often wished for a
magical combinator of type
(Monad m) => ((a -> b) -> c) -> (a -> m b) -> m c
which would let me inject a monadic function into a pure one, then
wrap the ultimate result to ensure that no nastiness escapes.
If the signature would be
(Monad m) => ((a -> b) -> c) -> m (a -> b) -> m c
it would be possible, and the implementation would be 'liftM'/'fmap'.
In the Reader monad you can even convert
(a -> m b) to m (a -> b)
The existence of a conversion
convert :: (a -> m b) -> m (a -> b)
between those two types for a given monad m is equivalent to the
existence of
magic :: ((a -> b) -> c) -> (a -> m b) -> m c
since we have
convert = magic id
magic f g = return f `ap` convert g
Regards,
apfelmus
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