[Haskell-cafe] Re: monad subexpressions

Brandon Michael Moore brandon at heave.ugcs.caltech.edu
Fri Aug 3 20:48:18 EDT 2007

<much snipping>

> Also, note, if you use the operators in Control.Applicative, then:
>   return $ foo $(bar1) $(bar2) $(bar3) ...
> can be:
>   return foo <*> bar1 <*> bar2 <*> bar3 ...
> or:
>   foo <$> bar1 <*> bar2 <*> bar3
> I don't (personally) see how that's any more cryptic than placing brackets 
> around around the monadic values themselves. 
> ...

Seconded. The main difference with brackes is that the application to pure
values looks the same as normal application.

> To get outside the scope of idiom brackets/applicative, you'd need a use case 
> like:
>   if $(mexpr) then branch1 else branch2
> or (lest that be to easy):
>   case $(mexpr) of
>       p1 -> branch1
>       p2 -> branch2
>       ...
> In other words, something where you're not simply applying a pure function to 
> a bunch of monadic arguments. I can't say I've run into such patterns much 
> myself, but I've been told they're common in xmonad, and may be elsewhere.

General purpose brackets are overkill here. I would really like a simple
monadic case. What's so bad about

caseM mexpr of
  p1 -> branch1
  p2 -> branch2


(mexpr >>= \e -> case e of
  p1 -> branch1
  p2 -> branch2)

It's simple sugar for working with monadic code, much like do notation.
(indeed, it seems to plug a gap - we have do for sequencing, liftM and
so on for application, but no sugar for case discrimination)

It's a much simpler sort of thing than this fancy sugar for intermixing
code in various monads people have been talking about (so far it seems
assumed that one is just Identity...)


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