[Ur] Some utility functions to add to base libs?

[Edward Z. Yang]

Here are two that I found useful:

    fun mapXiM [m ::: (Type -> Type)] (_ : monad m) [a] [ctx ::: {Unit}] (f : int -> a -> m (xml ctx [] [])) : list a -> m (xml ctx [] []) =
        let
            fun mapXiM' i ls =
                case ls of
                    [] => return <xml/>
                  | x :: ls =>
                    this <- f i x;
                    rest <- mapXiM' (i+1) ls;
                    return <xml>{this}{rest}</xml>
        in
            mapXiM' 0
        end

    fun liftM2 [m ::: (Type -> Type)] (_ : monad m) [a] [b] [c] (f : a -> b -> c) (mx : m a) (my : m b) : m c =
        x <- mx;
        y <- my;
        return (f x y)

It would also be nice for us Haskell folks if we had a 'val liftM = mp' somewhere too :-)

Edward