# Linearity and Erasure in Idris 2

I'm now actively working on a new implementation of Idris, Idris 2, and although there's still a long way to go, it's making very good progress. The documentation, as you see, is still essentially a placeholder, and there's no release yet. Nevertheless, now seems as good a time as any to start writing about some of the changes since Idris 1, some of the new features, and some of the improvements. Maybe I'll copy and paste it into the official documentation later :).

The biggest difference (internally) between Idris 1 and Idris 2 is that Idris 2 is
based on Quantitative Type Theory (QTT), a core language
developed by Bob Atkey and Conor McBride. In practice, this means that every
variable in Idris 2 has a *quantity* associated with it. A quantity is one of:

`0`, meaning that the variable is*erased*at run time`1`, meaning that the variable is used*exactly once*at run time*Unrestricted*, which is the same behaviour as Idris 1

Multiplicities are written in function types as follows:

`ignoreN : (0 n : Nat) -> Vect n a -> Nat`- this function cannot look at`n`at run time`duplicate : (1 x : a) -> (a, a)`- this function must use`x`exactly once (so good luck implementing it, by the way. There is no implementation because it would need to use`x`twice!)

If there is no multiplicity annotation, the argument is unrestricted.
If, on the other hand, a name is implicitly bound (like `a` in both examples above)
the argument is erased. So, the above types could also be written as:

`ignoreN : {0 a : _} -> (0 n : Nat) -> Vect n a -> Nat``duplicate : {0 a : _} -> (1 x : a) -> (a, a)`

In the rest of this post, I'll describe what this means for your Idris programs, with several examples. In particular, I'll describe:

- Erasure - how to know what is relevant at run time and what is erased
- Linearity - using the type system to encode
*resource usage protocols* - Pattern matching on types - truly first class types

The most important of these for most programs, and the thing you'll need to know about if converting Idris 1 programs to work with Idris 2, is erasure. The most interesting, however, and the thing which gives Idris 2 much more expressivity, is linearity, so we'll start there.

## Linearity

The `1` multiplicity expresses that a variable must be used exactly once.
First, we'll see how this works on some small examples of functions and
data types, then see how it can be used to encode resource protocols.

Above, we saw the type of `duplicate`. Let's try to write it interactively,
and see what goes wrong. We'll start by giving the type and a skeleton
definition with a hole:

duplicate : (1 x : a) -> (a, a) duplicate x = ?help

Checking the type of a hole tells us the multiplicity of each variable in
scope. If we check the type of `?help` we'll see that we can't
use `a` at run time, and we have to use `x` exactly once:

Main> :t help 0 a : Type 1 x : a ------------------------------------- help : (a, a)

If we use `x` for one part of the pair...:

duplicate : (1 x : a) -> (a, a) duplicate x = (x, ?help)

...then the type of the remaining hole tells us we can't use it for the other:

Main> :t help 0 a : Type 0 x : a ------------------------------------- help : a

The same happens if we try defining `duplicate x = (?help, x)` (try it!)
The intution behind multiplicity `1` is that if we have a function with
a type of the following form...:

f : (1 x : a) -> b

...then the guarantee given by the type system is that *if* `f x` *is used
exactly once, then* `x` *is used exactly once*. So, if we insist on
trying to define `duplicate`...:

duplicate x = (x, x)

...then Idris will complain:

pmtype.idr:2:15--8:1:While processing right hand side of Main.duplicate at pmtype.idr:2:1--8:1: There are 2 uses of linear name x

A similar intuition applies for data types. Consider the following types,
`Lin` which wraps an argument that must be used once, and `Unr` which
wraps an argument with unrestricted use:

data Lin : Type -> Type where MkLin : (1 x : a) -> Lin a data Unr : Type -> Type where MkUnr : (x : a) -> Unr a

If `MkLin x` is used once, then `x` is used once. But if `MkUnr x` is
used once, there is no guarantee on how often `x` is used. We can see this a
bit more clearly by starting to write projection functions for `Lin` and
`Unr` to extract the argument:

getLin : (1 val : Lin a) -> a getLin (MkLin x) = ?howmanyLin getUnr : (1 val : Unr a) -> a getUnr (MkUnr x) = ?howmanyUnr

Checking the types of the holes shows us that, for `getLin`, we must use
`x` exactly once (Because the `val` argument is used once,
by pattern matching on it as `MkLin x`, and if `MkLin x` is used once,
`x` must be used once):

Main> :t howmanyLin 0 a : Type 1 x : a ------------------------------------- howmanyLin : a

For `getUnr`, however, we still have to use `val` once, again by pattern
matching on it, but using `MkUnr x` once doesn't place any restrictions on
`x`. So, `x` has unrestricted use in the body of `getUnr`:

Main> :t howmanyUnr 0 a : Type x : a ------------------------------------- howmanyUnr : a

If `getLin` has an unrestricted argument...:

getLin : (val : Lin a) -> a getLin (MkLin x) = ?howmanyLin

...then `x` is unrestricted in `howmanyLin`:

Main> :t howmanyLin 0 a : Type x : a ------------------------------------- howmanyLin : a

Remember the intuition from the type of `MkLin` is that if `MkLin x` is
used exactly once, `x` is used exactly once. But, we didn't say that
`MkLin x` would be used exactly once, so there is no restriction on `x`.

### Resource protocols

One way to take advantage of being able to express linear usage of an argument
is in defining resource usage protocols, where we can use linearity to ensure
that any unique external resource has only one instance, and we can use
functions which are linear in their arguments to represent state transitions on
that resource. A door, for example, can be in one of two states, `Open` or
`Closed`:

data DoorState = Open | Closed data Door : DoorState -> Type where MkDoor : (doorId : Int) -> Door st

(Okay, we're just pretending here - imagine the `doorId` is a reference
to an external resource!)

We can define functions for opening and closing the door which explicitly describe how they change the state of a door, and that they are linear in the door:

openDoor : (1 d : Door Closed) -> Door Open closeDoor : (1 d : Door Open) -> Door Closed

Remember, the intuition is that if `openDoor d` is used exactly once,
then `d` is used exactly once. So, provided that a door `d` has
multiplicity `1` when it's created, we *know* that once we call
`openDoor` on it, we won't be able to use `d` again. Given that
`d` is an external resource, and `openDoor` has changed it's state,
this is a good thing!

We can ensure that any door we create has multiplicity `1` by
creating them with a `newDoor` function with the following type:

newDoor : (1 p : (1 d : Door Closed) -> IO ()) -> IO ()

That is, `newDoor` takes a function, which it runs exactly once. That
function takes a door, which is used exactly once. We'll run it in
`IO` to suggest that there is some interaction with the outside world
going on when we create the door. Since the multiplicity `1` means the
door has to be used exactly once, we need to be able to delete the door
when we're finished:

deleteDoor : (1 d : Door Closed) -> IO ()

So an example correct door protocol usage would be:

doorProg : IO () doorProg = newDoor $ \d => let d' = openDoor d d'' = closeDoor d' in deleteDoor d''

It's instructive to build this program interactively, with holes along
the way, and see how the multiplicities of `d`, `d'` etc change. For
example:

doorProg : IO () doorProg = newDoor $ \d => let d' = openDoor d in ?whatnow

Checking the type of `?whatnow` shows that `d` is now spent, but we
still have to use `d'` exactly once:

Main> :t whatnow 0 d : Door Closed 1 d' : Door Open ------------------------------------- whatnow : IO ()

Note that the `0` multiplicity for `d` means that we can still *talk*
about it - in particular, we can still reason about it in types - but we
can't use it again in a relevant position in the rest of the program.
It's also fine to shadow the name `d` throughout:

doorProg : IO () doorProg = newDoor $ \d => let d = openDoor d d = closeDoor d in deleteDoor d

If we don't follow the protocol correctly - create the door, open it, close it, then delete it - then the program won't type check. For example, we can try not to delete the door before finishing:

doorProg : IO () doorProg = newDoor $ \d => let d' = openDoor d d'' = closeDoor d' in putStrLn "What could possibly go wrong?"

This gives the following error:

Door.idr:15:19--15:38:While processing right hand side of Main.doorProg at Door.idr:13:1--17:1: There are 0 uses of linear name d''

There's a lot more to be said about the details here! But, this shows at
a high level how we can use linearity to capture resource usage protocols
at the type level. If we have an external resource which is guaranteed to
be used linearly, like `Door`, we don't need to run operations on that
resource in an `IO` monad, since we're already enforcing an ordering on
operations and don't have access to any out of date resource states. This is
similar to the way interactive programs work in
the Clean programming language, and in
fact is how `IO` is implemented internally in Idris 2, with a special
`%World` type for representing the state of the outside world that is
always used linearly:

public export data IORes : Type -> Type where MkIORes : (result : a) -> (1 x : %World) -> IORes a export data IO : Type -> Type where MkIO : (1 fn : (1 x : %World) -> IORes a) -> IO a

Having multiplicities in the type system raises several interesting questions, such as:

- Can we use linearity information to inform memory management and, for example, have type level guarantees about functions which will not need to perform garbage collection?
- How should multiplicities be incorporated into interfaces such as
`Functor`,`Applicative`and`Monad`? - If we have
`0`, and`1`as multiplicities, why stop there? Why not have`2`,`3`and more (like Granule) - What about multiplicity polymorphism, as in the Linear Haskell proposal?
- Even without all of that, what can we do
*now*?

I don't know the answer to any of these yet, but it's going to be fun finding out!

## Erasure

The `1` multiplicity give us many new possibilities in the kinds of
properties we can express. But, the `0` multiplicity is perhaps more
important in that it allows us to be precise about which values are
relevant at run time, and which are compile time only (that is, which are
erased). Using the `0` multiplicity means a function's type now tells us
exactly what it needs at run time.

For example, in Idris 1 you could get the length of a vector as follows:

vlen : Vect n a -> Nat vlen {n} xs = n

This is fine, since it runs in constant time, but the trade off is that
`n` has to be available at run time, so at run time we always need the length
of the vector to be available if we ever call `vlen`. Idris 1 can infer whether
the length is needed, but there's no easy way for a programmer to be sure.

In Idris 2, we'd need to state explicitly that `n` is needed at run time:

vlen : {n : Nat} -> Vect n a -> Nat vlen xs = n

(Incidentally, also note that in Idris 2, names bound in types are also available in the definition without explicitly rebinding them.)

This also means that when you call `vlen`, you need the length available. For
example, this will give an error:

sumLengths : Vect m a -> Vect n a —> Nat sumLengths xs ys = vlen xs + vlen ys

Idris 2 reports:

vlen.idr:7:20--7:28:While processing right hand side of Main.sumLengths at vlen.idr:7:1--10:1: m is not accessible in this context

This means that it needs to use `m` as an argument to pass to `vlen xs`,
where it needs to be available at run time, but `m` is not available in
`sumLengths` because it has multiplicity `0`.

We can see this more clearly by replacing the right hand side of
`sumLengths` with a hole...:

sumLengths : Vect m a -> Vect n a -> Nat sumLengths xs ys = ?sumLengths_rhs

...then checking the hole's type at the REPL:

Main> :t sumLengths_rhs 0 n : Nat 0 a : Type 0 m : Nat ys : Vect n a xs : Vect m a ------------------------------------- sumLengths_rhs : Nat

Instead, we need to give bindings for `m` and `n` with
unrestricted multiplicity:

sumLengths : {m, n : _} -> Vect m a -> Vect n a —> Nat sumLengths xs ys = vLen xs + vLen xs

Remember that giving no multiplicity on a binder, as with `m` and `n` here,
means that the variable has unrestricted usage.

(By the way, for any readers thinking "oh no, why does it have to be vectors all the time?" - I make no apology for using extremely familiar examples to introduce an unfamiliar new concept!)

If you're converting Idris 1 programs to work with Idris 2, this is probably the biggest thing you need to think about - along with the Idris 2 Prelude being much thinner, which is a topic I'll save for another time. It is important to note, though, that if you have bound implicits, such as...:

excitingFn : {t : _} -> Coffee t -> Moonbase t

...then it's a good idea to make sure `t` really is needed, or performance
might suffer due to the run time building the instance of `t` unnecessarily!

One final note on erasure: it is an error to try to pattern match on an
argument with multiplicity `0`, unless its value is inferrable from
elsewhere. So, the following definition is rejected:

badNot : (0 x : Bool) -> Bool badNot False = True badNot True = False

This is rejected with the error:

badnot.idr:2:1--3:1:Attempt to match on erased argument False in Main.badNot

The following, however, is fine, because in `sNot`, even though we appear
to match on the erased argument `x`, its value is uniquely inferrable from
the type of the second argument:

data SBool : Bool -> Type where SFalse : SBool False STrue : SBool True sNot : (0 x : Bool) -> SBool x -> Bool sNot False SFalse = True sNot True STrue = False

My own experience so far suggests that, most of the time, as long as you're using unbound implicits in your Idris 1 programs, they will work without much modification in Idris 2. The Idris 2 type checker will point out where you require an unbound implicit argument at run time - sometimes this is both surprising and enlightening!

## Pattern Matching on Types

One way to think about dependent types is to think of them as "first class" objects in the language, in that they can be assigned to variables, passed around and returned from functions, just like any other construct. But, if they're truly first class, we should be able to pattern match on them too! Idris 2 allows us to do this. For example:

showType : Type -> String showType Int = "Int" showType (List a) = "List of " ++ showType a showType _ = "something else"

We can try this as follows:

Main> showType Int "Int" Main> showType (List Int) "List of Int" Main> showType (List Bool) "List of something else"

Pattern matching on function types is interesting, because the return type
may depend on the input value. For example, let's add a case to
`showType`:

showType (Nat -> a) = ?help

Inspecting the type of `help` tells us:

Main> :t help a : Nat -> Type ------------------------------------- help : String

So, the return type `a` depends on the input value of type `Nat`, and
we'll need to come up with a value to use `a`, for example:

showType (Nat -> a) = "Function from Nat to " ++ showType (a Z)

Note that multiplicities on the binders, and the ability to pattern match
on *non-erased* types mean that the following two types are distinct:

id : a -> a notId : {a : Type} -> a -> a

In the case of `notId`, we can match on `a` and get a function which
is certainly not the identity function:

notId {a = Int} x = x + 1 notId x = x Main> notId 93 94 Main> notId "???" "???"

There is an important consequence of being able to distinguish between relevant
and irrelevant type arguments, which is that a function is *only* parametric in
`a` if `a` has multiplicity `0`. So, in the case of `notId`, `a` is
*not* a parameter, and so we can't draw any conclusions about the way the
function will behave because it is polymorphic, because the type tells us it
might pattern match on `a`.

On the other hand, it is merely a coincidence that, in non-dependently typed
languages, types are *irrelevant* and get erased, and values are *relevant*
and remain at run time. Idris 2, being based on QTT, allows us to make the
distinction between relevant and irrelevant arguments precise. Types can be
relevant, values (such as the `n` index to vectors) can be irrelevant.

That's all for now. I hope this is enough to get started with dealing with multiplicities in Idris 2!