Field labels that do updates

27 Feb 2009

      I have written a couple of small, experimental virtual machines in Haskell,
and I always use the State monad with the virtual machine data type as the
state.
   data VM a = VM { getAlpha :: Int , getBeta :: String , getGamma :: a }
which is all well and good, but I inevitably end up writing code like this
along with it:
    putAlpha a (VM _ b c) = (VM a b c)
    putBeta  b (VM a _ c) = (VM a b c)
    putGamma  c (VM a b _) = (VM a b c)
Its useful because you can just create one monadic function that updates the
state and pass one of the "put" functions as a parameter.
    updateVM :: (x -> VM a -> VM b) -> x -> State (VM b) ()
    updateVM  putFunc value = do { state <- get ; put (putFunc value state)
}

...some algorithm...
    do updateVM putAlpha 12
       updateVM putBeta "Hello"
       return somthing

But writing the "put" functions become tedious for virtual machines with
more fields in their type, especially if you need to add a field to the data
type in the future. Could there be syntactic sugar added to generate a list
of functions that update the fields of a data type?
   data VM a = VM { getAlpha/putAlpha :: Int , getBeta/putBeta :: String ,
getGamma/putGamma :: a }
Where the slash operator is optional, but if included in the code will cause
the compiler to generate functions of the given names that update those
fields.

Pros: one more time-saving feature implemented in syntactic sugar. The
optional nature of the slash operator would give users a choice of whether
or not to use it.
Cons: increases complexity of the syntax

I couldn't find such a suggestion on the mailing list, but something tells
me this idea is too simple to have not been suggested before. Sorry if this
is a redundant feature request.

Ramin Honary

Bas van Dijk

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