## INLINE Phases

A missing inline or inline in an incorrect GHC simplifier phase can adversely
impact performance.  We use three builtin phases of GHC simplifier for inlining
i.e. phase 0, 1 and 2. We have defined them as follows in `inline.h`:

```
#define INLINE_EARLY  INLINE [2]
#define INLINE_NORMAL INLINE [1]
#define INLINE_LATE   INLINE [0]
```

## Low Level `fromStreamD/toStreamD` Fusion

The combinators in `Streamly.Prelude` are defined in terms of combinators in
`Streamly.Internal.Data.Stream.StreamD` (Direct style streams) or `Streamly.Internal.Data.Stream.StreamK`
(CPS style streams). We convert the stream from `StreamD` to `StreamK`
representation or vice versa in some cases. 

In the first inlining phase (INLINE_EARLY or INLINE) we expand
the combinators in `Streamly.Prelude` into
fromStreamD/fromStreamK/toStreamD/toStreamK and combinators defined in StreamD
or StreamK modules. Once we do that fromStreamD/toStreamD get exposed and we
can apply rewrite rules to rewrite transformations like `fromStreamK .
toStreamK` to `id`. A plain `INLINE` pragma is usually enough on combinators in
`Streamly.Prelude`.

```
{-# RULES "fromStreamK/toStreamK fusion"
  forall s. toStreamK (fromStreamK s) = s #-}
```

Also, we have to prevent fromStreamK and toStreamK themselves from inlining in
this phase so that rewrite rules can be applied on them, therefore, we annotate
these functions with `INLINE_LATE`.

## Fallback Rules

In some cases, if the operation could not fuse we want to use a fallback
rewrite rule in the next phase. For such cases we use the INLINE_EARLY phase
for the first rewrite and the INLINE_NORMAL phase for the fallback rules.

The fallback rules make sure that if we could not fuse the direct style
operations then better use the CPS style operation, because unfused direct
style would have worse performance than the CPS style ops.

```
{-# INLINE_EARLY unfoldr #-}
unfoldr :: (Monad m, IsStream t) => (b -> Maybe (a, b)) -> b -> t m a
unfoldr step seed = fromStreamS (S.unfoldr step seed)
{-# RULES "unfoldr fallback to StreamK" [1]
     forall a b. S.toStreamK (S.unfoldr a b) = K.unfoldr a b #-}
```

## High Level Operation Fusion

Since each high level combinator in `Streamly.Prelude` is wrapped in
`fromStreamD/toStreamD` etc. the combinator fusion cannot work unless we have
removed those and exposed consecutive operations e.g. a `map` followed by
another `map`.  Assuming that redundant `fromStreamK/toStreamK` have been
removed in the `INLINE_EARLY` phase, we can then apply the combinator fusion
rules in the `INLINE_NORMAL` phase.  For example, we can fuse two `map`
operations into a single `map` operation.  Note that now we have exposed the
`StreamD/StreamK` implementations of combinators and the rules would apply on
those.

## Inlining Higher Order Functions

Note that partially applied functions cannot be inlined. So if we have a code
like this:

```
concatMap1 src = runStream $ S.concatMap (S.replicate 3) src
```

We want to ensure that `concatMap` gets inlined before `replicate` so that
`replicate` becomes fully applied before it gets inlined. Currently ensuring
that both of them are inlined in the same phase (`INLINE_NORMAL`) seems to be
enough to achieve that. In general, we should try to ensure that higher order
functions are inlined before or in the same phase as the functions they can
consume as arguments. This means `StreamD` combinators should not be marked
as `INLINE` or `INLINE_EARLY`, instead they should all be marked as
`INLINE_NORMAL` because higher order funcitons like `concatMap`/`map`/`mapM`
etc are marked as `INLINE_NORMAL`. `StreamD` functions in other modules like
`Streamly.Memory.Array` should also follow the same rules.

## Stream Fusion

In StreamD combinators, inlining the inner step or loop functions too early
i.e. in the same pahse or before the outer function is inlined may block stream
fusion opportunities. Therefore, the inner step functions and folding loops are
marked as INLINE_LATE.

## Specialization

In some cases, the `step` function in `StreamD` does not get specialized when
inlined unless it is provided with an explicit signature or made a lambda, for
example, in the `replicate/replicateM` combinator we need the type annotation
on `i` to get it specialized:

```
    {-# INLINE_LATE step #-}
    step _ (i :: Int) =
        if i <= 0
        then return Stop
        else do
                x <- action
                return $ Yield x (i - 1)
```

`-flate-specialise` also helps in this case.

## Stream and Fold State Data Structures

Since state is an internal data structure threaded around in the loop, it is a
good practice to use strict unboxed fields for state data structures where
possible. In most cases it is not necessary, but in some cases it may affect
fusion and make a difference of 10x performance or more.  For example, using
non-strict fields can increase the code size for internal join points and
functions created during transformations, which can affect the inlining of
these code blocks which in turn can affect stream fusion. 

See https://gitlab.haskell.org/ghc/ghc/issues/17075 .