MapWith alternatives and similar packages
Based on the "Combinators" category
Do you think we are missing an alternative of MapWith or a related project?
fmap, but can "inject" additional parameters such as whether first (or last) element, etc.
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 (
mapWithworks on all
- 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.
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.
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?
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
CurryTFas an application of a single value
7, but by
Data.Tuple.Curryas two values:
(), which I think is slightly more confusing than it needs to be.)