MapWith-0.2.0.0: README.md
# MapWith [](https://www.stackage.org/package/MapWith) [](https://hackage.haskell.org/package/MapWith) [](https://travis-ci.com/github/davjam/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](https://stackoverflow.com/questions/14114011/haskell-map-operation-with-different-first-and-last-functions#answer-53282575),
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](doc/examples.hs).
# 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](https://stackoverflow.com/questions/63504127/haskell-pinned-or-stack-memory-for-performance)).
- 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.)