[Haskell-cafe] functional update

Stuart Cook scook0 at gmail.com
Mon Apr 21 22:50:56 EDT 2008

On Tue, Apr 22, 2008 at 4:12 AM, Evan Laforge <qdunkan at gmail.com> wrote:
>  Has there been any work on improving update syntax in haskell?
>  Possibly some improvement could be made with a typeclass or two and a
>  few custom operators, to unify some of the disparate syntax.  Maybe
>  more improvement could be made with some TH hackery.  A better record
>  update syntax I'm sure could improve things even more.  Or maybe
>  there's a way to structure existing code to improve the above?

Here are two little TH functions I find useful:

-- \f x -> x { field = f (field x) }
alter :: Name -> Q Exp
alter field = do
    f <- newName "f"
    x <- newName "x"
    lamE [varP f, varP x] $
        recUpdE (varE x)
            [return (field, AppE (VarE f) (AppE (VarE field) (VarE x)))]

-- \a x -> x { field = a }
set :: Name -> Q Exp
set field = do
    a <- newName "a"
    x <- newName "x"
    lamE [varP a, varP x] $
        recUpdE (varE x)
            [return (field, VarE a)]

They're not as flexible as FRefs (though they could be helpful in
manually defining FRefs), but they still solve some of my frustrations
with record update syntax.

  $(set 'fieldname) value record -- sets the value of the "fieldname"
field to "value"
  $(alter 'fieldname) f record -- produces a new record where
"fieldname" has been transformed by f


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