MapWith

mapWith: like fmap, but can “inject” additional parameters such as whether first (or last) element, etc.

Background

I often want to map over a list, but do something slightly different with the first or last element.

For a long time I used markbounds, but also wanted something that:

• works on structures other than lists (mapWith works on all Traversable types);
• can provide additional types of parameter (not just first/last), such as:
  • index from start/end;
  • the previous/next element; and
• makes it easy to create new types of parameter to provide; and
• can provide any number of separate parameters to a function (not just a 3-tuple).

So, after only 2 years, I built a small library to do all of these.

Examples

Passing a “standard combination” of isFirst and isLast parameters:

let g x f l = [star f, x, star l]; star b = if b then '*' else ' '
in withFirstLast g "fred"
["*f ", " r ", " e ", " d*"]

Passing a custom combination of different types of parameter (the index from the start, whether it’s the last element, and elements from another list applied from the right):

let g x n l e = concat [[x], show n, if l then "*" else "-", e]
in mapWith (g ^-> eltIx & isLast <-^ eltFrom ["x","yy","z","zzzz","y"]) "fred"
["f0-zzzz","r1-z","e2-yy","d3*x"]

More examples are here.

Questions/Doubts

Note that this is my first library and my first use of cabal, so I’ve probably done some dumb things.

Some things I wonder:

• Doesn’t this already exist? (It feels like it should!)
• Should I name it Data.Traversable.MapWith? Or are such names “reserved” for “official” libraries, or something? Would this name impact my own file/directory structures?

Future Work

Areas for potential improvement in later releases:

• Performance investigations and hopefully improvements, in particular:

  • fusion for eltFrom Injectors (unlikely, given the reasons it’s not possible for zipWith, but we’ll see).
  • enhancements for “stateful” “from the right” Injectors (unlikely, given this).

• CurryTF: avoid tuples? (The tuple (7, ()) is interpreted by CurryTF as an application of a single value 7, but by Data.Tuple.Curry as two values: 7 and (), which I think is slightly more confusing than it needs to be.)