Make it possible to evaluate monadic actions when assigning record fields (<-)

[adde@…]

It is currently not possible to build records from values resulting from monadic actions while still using the field-specifiers.

foo :: IO Int ...

data Bar = Bar {

barFoo

Int

}

buildBar :: IO () buildBar = do

return Bar {

barFoo <- foo --Evaluate foo to get the Int-value

}

I've found two possible ways of doing this:

1) Using temporaries to evaluate the actions before assigning which doubles the number of lines: tmp <- foo return Bar {

barFoo = tmp

}

2) Lifting the record constructor which prevents you from using field specifiers (and you really need field specifiers when dealing with larger records): liftM Bar foo

[chevalier@…]

If you wrote:

return Bar {
 barFoo <- foo
 barQuux <- quux
}

which do you think should be executed first -- computation foo or computation quux?

[igloo]

This is being discussed on the Haskell' list:  http://www.haskell.org/pipermail/haskell-prime/2007-July/002265.html

[morabbin]

Bump; still relevant? Seems a bit iffy to me.

[liyang]

Is this really necessary now that we have RecordWildCards?, NamedFieldPuns? and Applicative? (All of which were mentioned in the Haskell' discussion.)