Make it possible to evaluate monadic actions when assigning record fields

[Adde]

On Tue, 2007-07-10 at 17:04 +0000, kahl at cas.mcmaster.ca wrote:
> Isaac Dupree <isaacdupree at charter.net> 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