I originally wrote this content as a chapter of Mezzo Haskell. I'm going to be starting up a similar effort to Mezzo Haskell in the next few days, and I wanted to get a little more attention on this content to get feedback on style and teaching approach. I'll be discussing that new initiative on the Commercial Haskell mailing list.

The point of this chapter is to help you peel back some of the layers of abstraction in Haskell coding, with the goal of understanding things like primitive operations, evaluation order, and mutation. Some concepts covered here are generally "common knowledge" in the community, while others are less well understood. The goal is to cover the entire topic in a cohesive manner. If a specific section seems like it's not covering anything you don't already know, skim through it and move on to the next one.

While this chapter is called "Primitive Haskell," the topics are very much GHC-specific. I avoided calling it "Primitive GHC" for fear of people assuming it was about the internals of GHC itself. To be clear: these topics apply to anyone compiling their Haskell code using the GHC compiler.

Note that we will not be fully covering all topics here. There is a "further reading" section at the end of this chapter with links for more details.

Let's start with a really simple question: tell me how GHC deals
with the expression `1 + 2`

. What *actually*
happens inside GHC? Well, that's a bit of a trick question, since
the expression is polymorphic. Let's instead use the more concrete
expression `1 + 2 :: Int`

.

The `+`

operator is actually a method of
the `Num`

type class, so we need to look at
the `Num Int`

instance:

instance Num Int where I# x + I# y = I# (x +# y)

Huh... well *that* looks somewhat magical. Now we need to
understand both the `I#`

constructor and the
`+#`

operator (and what's with the hashes all of a
sudden?). If we do a
Hoogle search, we can easily
find the relevant docs, which leads us to the following
definition:

data Int = I# Int#

So our first lesson: the `Int`

data type you've been
using since you first started with Haskell isn't magical at all,
it's defined just like any other algebraic data type... except for
those hashes. We can also search
for `+#`

, and end up at
some documentation giving the type:

+# :: Int# -> Int# -> Int#

Now that we know all the types involved, go back and look at the
`Num`

instance I quoted above, and make sure you feel
comfortable that all the types add up (no pun intended). Hurrah, we
now understand exactly how addition of `Int`

s works.
Right?

Well, not so fast. The Haddocks for `+#`

have a very
convenient source link... which (apparently due to a Haddock bug)
doesn't actually work. However, it's easy enough
to find the correct hyperlinked source. And now we see the
implementation of `+#`

, which is:

infixl 6 +# (+#) :: Int# -> Int# -> Int# (+#) = let x = x in x

That doesn't look like addition, does it? In fact, ```
let x =
x in x
```

is another way of saying bottom, or
`undefined`

, or infinite loop. We have now officially
entered the world of primops.

primops, short for primary operations, are core pieces of
functionality provided by GHC itself. They are the magical boundary
between "things we do in Haskell itself" and "things which our
implementation provides." This division is actually quite elegant;
as we already explored, the standard `+`

operator and
`Int`

data type you're used to are actually themselves
defined in normal Haskell code, which provides many benefits: you
get standard type class support, laziness, etc. We'll explore some
of that in more detail later.

Look at
the implementation of other functions in `GHC.Prim`

;
they're *all* defined as `let x = x in x`

. When GHC
reaches a call to one of these primops, it automatically replaces
it with the real implementation for you, which will be some
assembly code, LLVM code, or something similar.

Why do all of these functions end in a `#`

? That's
called the magic hash (enabled by the `MagicHash`

language extension), and it is a convention to distinguish boxed
and unboxed types and operations. Which, of course, brings us to
our next topic.

The `I#`

constructor is actually just a normal data
constructor in Haskell, which happens to end with a magic hash.
However, `Int#`

is *not* a normal Haskell data
type. In `GHC.Prim`

, we can see that it's implementation
is:

data Int#

Which, like everything else in `GHC.Prim`

is really a
lie. In fact, it's provided by the implementation, and is in fact a
normal `long int`

from C (32-bit or 64-bit, depending on
architecture). We can see something even funnier about it in
GHCi:

```
> :k Int
Int :: *
> :k Int#
Int# :: #
```

That's right, `Int#`

has a different *kind* than
normal Haskell datatypes: `#`

. To quote
the GHC docs:

Most types in GHC are boxed, which means that values of that type are represented by a pointer to a heap object. The representation of a Haskell

`Int`

, for example, is a two-word heap object. An unboxed type, however, is represented by the value itself, no pointers or heap allocation are involved.

See those docs for more information on distinctions between boxed and unboxed types. It is vital to understand those differences when working with unboxed values. However, we're not going to go into those details now. Instead, let's sum up what we've learnt so far:

`Int`

addition is just normal Haskell code in a typeclass`Int`

itself is a normal Haskell datatype- GHC provides
`Int#`

and`+#`

as an unboxed`long int`

and addition on that type, respectively. This is exported by`GHC.Prim`

, but the real implementation is "inside" GHC. - An
`Int`

contains an`Int#`

, which is an unboxed type. - Addition of
`Int`

s takes advantage of the`+#`

primop.

Alright, we understand basic addition! Let's make things a bit more complicated. Consider the program:

main = do let x = 1 + 2 y = 3 + 4 print x print y

We know for certain that the program will first print
`3`

, and then print `7`

. But let me ask you a
different question. Which operation will GHC perform first: ```
1
+ 2
```

or `3 + 4`

? If you guessed ```
1 +
2
```

, you're *probably* right, but not necessarily! Thanks
to referential transparency, GHC is fully within its rights to
rearrange evaluation of those expressions and add ```
3 +
4
```

before `1 + 2`

. Since neither expression
depends on the result of the other, we know that it is irrelevant
which evaluation occurs first.

Note: This is covered in much more detail on the GHC wiki's evaluation order and state tokens page.

That begs the question: if GHC is free to rearrange evaluation
like that, how could I say in the previous paragraph that the
program will always print `3`

before printing
`7`

? After all, it doesn't appear that ```
print
y
```

uses the result of `print x`

at all, so we not
rearrange the calls? To answer that, we again need to unwrap some
layers of abstraction. First, let's evaluate and inline
`x`

and `y`

and get rid of the
`do`

-notation sugar. We end up with the program:

main = print 3 >> print 7

We know that `print 3`

and `print 7`

each
have type `IO ()`

, so the `>>`

operator
being used comes from the `Monad IO`

instance. Before we
can understand that, though, we need to look at
the definition of `IO`

itself

newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

We have a few things to understand about this line. Firstly,
`State#`

and
`RealWorld`

. For now, just pretend like they are a
single type; we'll see when we get to `ST`

why
`State#`

has a type parameter.

The other thing to understand is that `(# ... #)`

syntax. That's an *unboxed tuple*, and it's a way of returning
multiple values from a function. Unlike a normal, boxed tuple,
unboxed tuples involve no extra allocation and create no
thunks.

So `IO`

takes a real world state, and gives you back
a real world state and some value. And that right there is how we
model side effects and mutation in a referentially transparent
language. You may have heard the description of `IO`

as
"taking the world and giving you a new one back." What we're doing
here is threading a specific state token through a series of
function calls. By creating a dependency on the result of a
previous function, we are able to ensure evaluation order, yet
still remain purely functional.

Let's see this in action, by coming back to our example from
above. We're now ready to look at
the `Monad IO`

instance:

instance Monad IO where (>>) = thenIO thenIO :: IO a -> IO b -> IO b thenIO (IO m) k = IO $ \ s -> case m s of (# new_s, _ #) -> unIO k new_s unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #)) unIO (IO a) = a

(Yes, I changed things a bit to make them easier to understand.
As an exercise, compare that this version is in fact equivalent to
what is actually defined in `GHC.Base`

.)

Let's inline these definitions into ```
print 3 >> print
7
```

:

main = IO $ \s0 -> case unIO (print 3) s0 of (# s1, res1 #) -> unIO (print 7) s1

Notice how, even though we ignore the *result* of
`print 3`

(the `res1`

value), we still depend
on the new state token `s1`

when we evaluate ```
print
7
```

, which forces the order of evaluation to first evaluate
`print 3`

and then evaluate `print 7`

.

If you look through `GHC.Prim`

, you'll see that a
number of primitive operations are defined in terms of ```
State#
RealWorld
```

or `State# s`

, which allows us to force
evaluation order.

**Exercise**: implement a function ```
getMaskingState ::
IO Int
```

using the `getMaskingState#`

primop and
the `IO`

data constructor.

Let's compare the definitions of the `IO`

and
`ST`

types:

newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #)) newtype ST s a = ST (State# s -> (# State# s, a #))

Well *that* looks oddly similar. Said more precisely,
`IO`

is isomorphic to `ST RealWorld`

.
`ST`

works under the exact same principles as
`IO`

for threading state through, which is why we're
able to have things like mutable references in the `ST`

monad.

By using an uninstantiated `s`

value, we can ensure
that we aren't "cheating" and running arbitrary `IO`

actions inside an `ST`

action. Instead, we just have
"local state" modifications, for some definition of local state.
The details of using `ST`

correctly and the Rank2Types
approach to `runST`

are interesting, but beyond the
scope of this chapter, so we'll stop discussing them here.

Since `ST RealWorld`

is isomorphic to
`IO`

, we should be able to convert between the two of
them. `base`

does in fact
provide the `stToIO`

function.

**Exercise**: write a pair of functions to convert between
`IO a`

and `ST RealWorld a`

.

**Exercise**: `GHC.Prim`

has a
section on mutable variables, which forms the basis on
`IORef`

and `STRef`

. Provide a new
implementation of `STRef`

, including
`newSTRef,`

readSTRef`, and`

writeSTRef`.

It's a bit unfortunate that we have to have two completely
separate sets of APIs: one for `IO`

and another for
`ST`

. One common example of this is `IORef`

and `STRef`

, but- as we'll see at the end of this
section- there are plenty of operations that we'd like to be able
to generalize.

This is where `PrimMonad`

, from the
`primitive`

package, comes into play. Let's look at
its definition:

-- | Class of primitive state-transformer monads class Monad m => PrimMonad m where -- | State token type type PrimState m -- | Execute a primitive operation primitive :: (State# (PrimState m) -> (# State# (PrimState m), a #)) -> m a

Note: I have *not* included the `internal`

method, since it will likely be
removed. In fact, at the time you're reading this, it may
already be gone!

`PrimState`

is an associated type giving the type of
the state token. For `IO`

, that's
`RealWorld`

, and for `ST s`

, it's
`s`

. `primitive`

gives a way to lift the
internal implementation of both `IO`

and `ST`

to the monad under question.

**Exercise**: Write implementations of the ```
PrimMonad
IO
```

and `PrimMonad (ST s)`

instances, and compare
against the real ones.

The primitive package provides a number of wrappers around types
and functions from `GHC.Prim`

and generalizes them to
both `IO`

and `ST`

via the
`PrimMonad`

type class.

**Exercise**: Extend your previous `STRef`

implementation to work in any `PrimMonad`

. After you're
done, you may want to
have a look at Data.Primitive.MutVar.

The `vector`

package builds on top of the
`primitive`

package to provide mutable vectors that can
be used from both `IO`

and `ST`

. This chapter
is *not* a tutorial on the `vector`

package, so we
won't go into any more details now. However, if you're curious,
please
look through the `Data.Vector.Generic.Mutable`

docs.

To tie this off, we're going to implement a
`ReaderIO`

type. This will flatten together the
implementations of `ReaderT`

and `IO`

.
Generally speaking, there's no advantage to doing this: GHC should
always be smart enough to generate the same code for this and for
`ReaderT r IO`

(and in my benchmarks, they perform
identically). But it's a good way to test that you understand the
details here.

You may want to try implementing this yourself before looking at the implementation below.

{-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MagicHash #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UnboxedTuples #-} import Control.Applicative (Applicative (..)) import Control.Monad (ap, liftM) import Control.Monad.IO.Class (MonadIO (..)) import Control.Monad.Primitive (PrimMonad (..)) import Control.Monad.Reader.Class (MonadReader (..)) import GHC.Base (IO (..)) import GHC.Prim (RealWorld, State#) -- | Behaves like a @ReaderT r IO a@. newtype ReaderIO r a = ReaderIO (r -> State# RealWorld -> (# State# RealWorld, a #)) -- standard implementations... instance Functor (ReaderIO r) where fmap = liftM instance Applicative (ReaderIO r) where pure = return (<*>) = ap instance Monad (ReaderIO r) where return x = ReaderIO $ \_ s -> (# s, x #) ReaderIO f >>= g = ReaderIO $ \r s0 -> case f r s0 of (# s1, x #) -> let ReaderIO g' = g x in g' r s1 instance MonadReader r (ReaderIO r) where ask = ReaderIO $ \r s -> (# s, r #) local f (ReaderIO m) = ReaderIO $ \r s -> m (f r) s instance MonadIO (ReaderIO r) where liftIO (IO f) = ReaderIO $ \_ s -> f s instance PrimMonad (ReaderIO r) where type PrimState (ReaderIO r) = RealWorld primitive f = ReaderIO $ \_ s -> f s -- Cannot properly define internal, since there's no way to express a -- computation that requires an @r@ input value as one that doesn't. This -- limitation of @PrimMonad@ is being addressed: -- -- https://github.com/haskell/primitive/pull/19 internal (ReaderIO f) = f (error "PrimMonad.internal: environment evaluated")

**Exercise**: Modify the `ReaderIO`

monad to
instead be a `ReaderST`

monad, and take an
`s`

parameter for the specific state token.

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