[Haskell-cafe] Re: monad subexpressions

Claus Reinke claus.reinke at talk21.com
Fri Aug 3 20:07:27 EDT 2007

>> mytransaction = foo `liftM` r xvar0 `ap` r xvar1 ..
>>     where r = readTVar
> I really find it difficult to articulate why this isn't acceptable,
> because it seems so obvious to me! It's short yes, but I really don't
> think it's very clear...

if it is any consolation, i don't use that style myself (yet?-). but
it is a useful stepping stone on a path that seems to go somewhat
like this:

    - explicit do-notation with flattened parameters
    - explicitly defined lifted operations
    - liftMn, for on-the-spot lifting 
    - liftM/ap (avoiding need for infinitely many liftMn)
    - idioms  http://www.cs.nott.ac.uk/~ctm/Idiom.pdf
    - idiom brackets
    - .. ?-)

> I have a hard time believing that anyone finds that natural. After
> lots and lots of mind-bending forays into various branches of
> mathematics, then yes maybe you can get used to it, but it's hardly as
> natural as saying "add this one symbol to your values to extract
> monadic values left-to-right".

what makes this unnatural to me is that it is built-in syntax, which
not only interacts badly with haskell's general abstraction facilities,
but is outside the programmers' control. once we've figured out
what we want, programatically, then putting a nice syntax on top
of it, that is syntactic sugar, but binding fairly complex syntax
transformations to an innocent-looking syntax is not.

perhaps a good start would be syntactic sugar for idiom brackets,
to rescue them from the complexities of type-level programming?
at least, that would be a local transformation with well-explored
semantics, similar to do-notation on top of >>=/return.

if that doesn't work out, one might take another look at (<-).


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