# justified-containers
Keyed container types with type-checked proofs of key presence.
# Description
Have you ever *known* that a key could be found in a certain map? Were you tempted to
reach for `fromJust` or `error` to handle the "impossible" case, when you knew that
`lookup` should give `Just v`? (and did shifting requirements ever make the impossible
become possible after all?)
`Data.Map.Justified` provides a zero-cost `newtype` wrapper around `Data.Map.Map`
that enables you to separate the *proof that a key is present* from the *operations using the key*.
Once you prove that a key is present, you can use it `Maybe`-free in any number of other
operations -- sometimes even operations on other maps!
None of the functions in this module can cause a run-time error, and very few
of the operations return a `Maybe` value.
See the `Data.Map.Justified.Tutorial` module for usage examples, or browse the API of the
most recent release [on Hackage](https://hackage.haskell.org/package/justified-containers/docs/Data-Map-Justified.html).
### Example
```haskell
withMap test_table $ \table -> do
case member 1 table of
Nothing -> putStrLn "Sorry, I couldn't prove that the key is present."
Just key -> do
-- We have proven that the key is present, and can now use it Maybe-free...
putStrLn ("Found key: " ++ show key)
putStrLn ("Value for key: " ++ lookup key table)
-- ...even in certain other maps!
let table' = reinsert key "howdy" table
putStrLn ("Value for key in updated map: " ++ lookup key table')
```
Output:
Found key: Key 1
Value for key: hello
Value for key in updated map: howdy
## Motivation: `Data.Map` and `Maybe` values
Suppose you have a key-value mapping using `Data.Map`'s type `Map k v`. Anybody making
use of `Map k v` to look up or modify a value must take into account the possibility
that the given key is not present.
In `Data.Map`, there are two strategies for dealing with absent keys:
1. Cause a runtime error (e.g. `Data.Map`'s `(!)` when the key is absent)
2. Return a `Maybe` value (e.g. `Data.Map`'s `lookup`)
The first option introduces partial functions, so is not very palatable. But what is
wrong with the second option?
To understand the problem with returning a `Maybe` value, let's ask what the `Maybe v` in
```haskell
lookup :: k -> Map k v -> Maybe v
```
really does for us. By returning
a `Maybe v` value, `lookup key table` is saying "Your program must account
for the possibility that `key` cannot be found in `table`. I will ensure that you
account for this possibility by forcing you to handle the `Nothing` case."
In effect, `Data.Map` is requiring the user to prove they have handled the
possibility that a key is absent whenever they use the `lookup` function.
## Laziness (the bad kind)
Every programmer has probably had the experience of knowing, somehow, that a certain
key is going to be present in a map. In this case, the `Maybe v` feels like a burden:
I already *know* that this key is in the map, why should I have to handle the `Nothing` case?
In this situation, it is tempting to reach for the partial function `fromJust`,
or a pattern match like `Nothing -> error "The impossible happened!"`. But as parts of
the program are changed over time, you may find the impossible has become possible after
all (or perhaps youll see the dreaded and unhelpful `*** Exception: Maybe.fromJust: Nothing`)
It is tempting to reach for partial functions or "impossible" runtime errors here, because
the programmer has proven that the key is a member of the map in some other way. They
know that lookup` should return a `Just v` --- but the *compiler* doesnt know this!
The idea behind `Data.Map.Justified` is to encode the programmers knowledge that a key
is present *within the type system*, where it can be checked at compile-time. Once a key
is known to be present, `Data.Map.Justified`'s `lookup` will never fail. Your justification
removes the `Just`!
# How it works
Evidence that a key can indeed be found in a map is carried by a phantom type parameter `ph`
shared by both the `Data.Map.Justified.Map` and `Data.Map.Justified.Key` types. If you are
able to get your hands on a value of type `Key ph k`, then you must have already proven that
the key is present in *any* value of type `Map ph k v`.
The `Key ph k` type is simply a `newtype` wrapper around `k`, but the phantom type `ph` allows
`Key ph k` to represent both *a key of type `k`* __and__ *a proof that the key is present in*
*all maps of type `Map ph k v`*.
There are several ways to prove that a key belongs to a map, but the simplest is to just use
`Data.Map.Justified`'s `member` function. In `Data.Map`, `member`
has the type
```haskell
member :: Ord k => k -> Map k v -> Bool
```
and reports whether or not the key can be found in the map. In `Data.Map.Justified`,
`member` has the type
```haskell
member :: Ord k => k -> Map ph k v -> Maybe (Key ph k)
```
Instead of a boolean, `Data.Map.Justified`'s `member` either says `the key is not present`
(`Nothing`), or gives back the same key, *augmented with evidence that they key*
*is present*. This key-plus-evidence can then be used to do any number of `Maybe`-free
operations on the map.
`Data.Map.Justified` uses the same rank-2 polymorphism trick used in the `Control.Monad.ST` monad to
ensure that the `ph` phantom type can not be extracted; in effect, the proof that a key is
present can't leak to contexts where the proof would no longer be valid.