# autoapply

A Template-Haskell program to automatically pass arguments to functions
wherever the type fits.

@tomjaguarpaw noticed that this is like "Type-directed implicit parameters".

## TL;DR

You have the following values and want to stir them together and see what
sticks.

- `foo :: Monad m => A -> B -> C -> m D`
- `getA :: App A`
- `myC :: C`

`$(autoApply [] ['getA, 'myC] 'foo)` will create
`\b -> getA >>= \a -> foo a b myC` which has type `B -> App D`

or

`autoApplyDecs reverse [] ['getA, 'myC] ['foo]` will create
`oof :: B -> App D; oof b = do { a <- getA; foo a b myC }`

## Why to use it

One nice use-case is to avoiding writing boilerplate wrappers for using an API
in your Monad stack. For instance imagine the following API.

```haskell
data Instance; data ExtraOpenInfo; data Foo; data Bar; data Handle
openHandle  :: MonadIO m => Instance -> Maybe ExtraOpenInfo -> m Handle
closeHandle :: MonadIO m => Instance -> Handle -> m ()
useHandle   :: MonadIO m => Instance -> Handle -> Foo -> m Bar
```

You'd like to use this in your `polysemy` application, using the `Input` effect
to pass the `Instance` handle around, and always passing `Nothing` for
`ExtraOpenInfo` because you don't use that functionality and getting a `Foo`
from some other constraint `MyConstraint`. You define the following values.

```haskell
myExtraOpenInfo :: Maybe ExtraOpenInfo
myExtraOpenInfo = Nothing
getInstance :: Member (Input Instance) r => Sem r Instance
getInstance = input
getFoo :: MyConstraint m => m Foo
getFoo = ...
```

You then create the wrapped API thusly:

```haskell
autoapplyDecs
  (<> "'") -- Function to transform the names of the wrapped functions
  ['myExtraOpenInfo, 'getInstance, 'getFoo]
    -- Potential arguments to pass which must subsume the argument type of the
    -- function.
  [] -- Potential arguments to pass which must unify with the argument type
  ['openHandle, 'closeHandle, 'useHandle] -- Functions to wrap
```

Which creates the following declarations:

```haskell
openHandle'
  :: (Member (Input Instance) r, MonadIO (Sem r)) => Sem r Handle
closeHandle'
  :: (Member (Input Instance) r, MonadIO (Sem r)) => Handle -> Sem r ()
useHandle'
  :: (Member (Input Instance) r, MyConstraint (Sem r), MonadIO (Sem r))
  => Handle -> Sem r Bar
```

Notice:
- `Instance` is supplied with the `Member (Input Instance) r` constraint
- `Foo` is supplied by `MyConstraint (Sem r)`
- `ExtraOpenInfo` is not present at all, being supplied internally by `myExtraOpenInfo`

To see the generated code (it's exactly what you'd expect) compile
`test/Types.hs` with `-ddump-splices`.

## How to use this

To generate a new top-level declaration you'll need:

- The `Name` of a function to apply to some arguments.
- The `Name`s of some values to try and pass as arguments
  - values whose type must subsume the argument type (if you're unsure, you
    probably want this one)
  - values whose type must merely unify with the argument type
- A way of generating a name for this declaration given the wrapped name
  `:: String -> String`.

The new declaration will be generated, equal to the wrapped one but using the
supplied arguments wherever possible.

Arguments can be used in two ways:

- As regular parameters
  - If the type of the argument matches directly
  - An example is applying `takeWhile` to `not`; `not` is passed as the `a -> Bool`
    argument to `takeWhile`. `$(autoapply [] ['not] 'takeWhile) :: [Bool] -> [Bool]`

- Using a monadic bind
  - If the wrapped function returns a value of type `m a` and there exists an instance `Monad m`
  - If the argument is of type `n a` and there exists an instance `Monad m`
  - If `m` unifies with `n`
  - An example is applying `putStrLn` to `getLine`. The `String` result of `getLine` is passed to `putStrLn`
    `$(autoapply [] ['getLine] 'putStrLn) :: IO ()`

It's important to note that `Monad` instance checking only goes as far as
`template-haskell`'s `reifyInstances`. i.e. only the instance heads are
checked.

Constraints are checked (up to `reifyInstances`) so it won't pass `reverse` to
`(+)` for example.

Monadic binds are performed in the order of arguments passed to the wrapped
function, and will be performed more than once if the argument is used multiple
times.

You may want to either type your generated declarations manually (putting the
type after the splice) or turn on `-XNoMonomorphismRestriction` if your
arguments have polymorphic constraints.

## Where to use it

- In an expression context:
  - `$(autoApply ['my, 'arguments] [] 'myFunction)`

- At the top level to generate several declarations
  - `$(autoApplyDecs (funNameToNewFunName :: String -> String) ['my] ['arguments] ['myFunction, 'anotherFunction])`

## See also

This has a similar feel to some other programs which also generate Haskell
expressions based on types.

- [djinn](https://hackage.haskell.org/package/djinn)
- [exference](http://hackage.haskell.org/package/exference) ([github](https://github.com/lspitzner/exference))
- [JustDoIt](https://www.joachim-breitner.de/blog/735-The_magic_%E2%80%9CJust_do_it%E2%80%9D_type_class)

There are a couple of differences here:

- One doesn't need to specify the desired type up front, this tool will just go
  as far as it can.
- This tool isn't doing any interesting proof search instead it's just "if it
  fits, I sits"