Yep, I do this often and it's pretty nasty. It's especially inconvenient that you don't have access to computations before the if, for example: do x <- something y <- something-else case y of Foo a -> do .... <--- I want access to 'x' here Bar b -> ... -deech On Wed, Jan 26, 2011 at 2:51 PM, Edward Amsden wrote:

Those functions are useful if I'm more interested in actions (for instance putStrLn) in IO, but they restrict the monad's return type to ().

I'm often writing code like the following:

do ... case ... of Foo x -> do ...                  ...                  return foobar Bar x -> do ...                 ...                 return foobar

or worse:

if cond  then throwError "BBQ!"  else do         ...         ...         return ...

(which works btw because throwError is polymorphic in the return type)

So I end up with a nasty set of nested do statements for case expressions.

On Wed, Jan 26, 2011 at 3:32 PM, aditya siram wrote:

...

I think that for monads the cleanest way of doing conditional execution is using  'when' and 'unless' [1] and 'guard' if your type is a Monoid.

-deech

[1] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.htm...

On Wed, Jan 26, 2011 at 2:22 PM, Edward Amsden wrote:

...

Inside a do block, I can very conveniently substitute

let x = <pure exp> <continued monadic code>

for either x <- return <pure exp> <continued monadic code>

or let x = <pure exp> in do <continued monadic code>

However, I can't do anything similar (that I know of) with if or case expressions. If I use if or case inside a do block, it's likely that I'm still using monadic expressions (at least in my experience). Is there some nasty semantic ambiguity that comes of this? Is it just not in the standard yet? Is there some extension that I'm not aware of?

-- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

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-- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

Excerpts from Edward Amsden's message of Wed Jan 26 15:51:46 -0500 2011:

do ... case ... of Foo x -> do ... ... return foobar Bar x -> do ... ... return foobar

There's not enough context here to say what is appropriate. Is foobar defined in the outer or inner do-block?

if cond then throwError "BBQ!" else do ... ... return ...

Try: when cond $ throwError "BBQ!" ... ... return ... Cheers, Edward