Re: Make it possible to evaluate monadic actions when assigning record fields

On Tue, 2007-07-10 at 17:04 +0000, kahl@cas.mcmaster.ca wrote:

Isaac Dupree wrote:

...

Adde wrote:

...

tmp <- foo return Bar { barFoo = tmp }

There is a feature being worked on in GHC HEAD that would let you do

do tmp <- foo return Bar{..}

which captures fields from everything of the same name that's in scope. I think this would also satisfy your desire.

I guess this means I could write:

data D = C {field1 :: Bool, field2 :: Char}

f x = do field1 <- foo1 field2 <- foo2 field3 <- foo3 other stuff return C{..}

instead of

f x = do tmp1 <- foo1 tmp2 <- foo2 field3 <- foo3 other stuff return $ C { field1 = tmp1, field2 = tmp2 }

This has a dangerous feel to it --- extending the definition of D to include a field field3 may have quite unintended consequences.

What I am missing most in the record arena is a functional notation for record update, for example:

{^ field1 } = \ f r -> r {field1 = f (field1 r)}

I agree, capturing variables without asking is just scary. While I'm pretty biased I still think my suggestion solves the problem in a cleaner, more consistent way. /Adde