The IO Monad

IO as a monad can be understood from two perspectives, which were described in the section on running programs. Each can help to understand the meanings of pure and bind for IO.

From the first perspective, an IO action is an instruction to Lean's run-time system. For example, the instruction might be "read a string from this file descriptor, then re-invoke the pure Lean code with the string". This perspective is an exterior one, viewing the program from the perspective of the operating system. In this case, pure is an IO action that does not request any effects from the RTS, and bind instructs the RTS to first carry out one potentially-effectful operation and then invoke the rest of the program with the resulting value.

From the second perspective, an IO action transforms the whole world. IO actions are actually pure, because they receive a unique world as an argument and then return the changed world. This perspective is an interior one that matches how IO is represented inside of Lean. The world is represented in Lean as a token, and the IO monad is structured to make sure that each token is used exactly once.

To see how this works, it can be helpful to peel back one definition at a time. The #print command reveals the internals of Lean datatypes and definitions. For example,

#print Nat

results in

inductive Nat : Type
number of parameters: 0
constructors: : Nat
Nat.succ : Nat → Nat


#print Char.isAlpha

results in

def Char.isAlpha : Char → Bool :=
fun c => Char.isUpper c || Char.isLower c

Sometimes, the output of #print includes Lean features that have not yet been presented in this book. For example,

#print List.isEmpty


def List.isEmpty.{u} : {α : Type u} → List α → Bool :=
fun {α} x =>
  match x with
  | [] => true
  | head :: tail => false

which includes a .{u} after the definition's name, and annotates types as Type u rather than just Type. This can be safely ignored for now.

Printing the definition of IO shows that it's defined in terms of simpler structures:

#print IO
@[reducible] def IO : Type → Type :=
EIO IO.Error

IO.Error represents all the errors that could be thrown by an IO action:

#print IO.Error
inductive IO.Error : Type
number of parameters: 0
IO.Error.alreadyExists : Option String → UInt32 → String → IO.Error
IO.Error.otherError : UInt32 → String → IO.Error
IO.Error.resourceBusy : UInt32 → String → IO.Error
IO.Error.resourceVanished : UInt32 → String → IO.Error
IO.Error.unsupportedOperation : UInt32 → String → IO.Error
IO.Error.hardwareFault : UInt32 → String → IO.Error
IO.Error.unsatisfiedConstraints : UInt32 → String → IO.Error
IO.Error.illegalOperation : UInt32 → String → IO.Error
IO.Error.protocolError : UInt32 → String → IO.Error
IO.Error.timeExpired : UInt32 → String → IO.Error
IO.Error.interrupted : String → UInt32 → String → IO.Error
IO.Error.noFileOrDirectory : String → UInt32 → String → IO.Error
IO.Error.invalidArgument : Option String → UInt32 → String → IO.Error
IO.Error.permissionDenied : Option String → UInt32 → String → IO.Error
IO.Error.resourceExhausted : Option String → UInt32 → String → IO.Error
IO.Error.inappropriateType : Option String → UInt32 → String → IO.Error
IO.Error.noSuchThing : Option String → UInt32 → String → IO.Error
IO.Error.unexpectedEof : IO.Error
IO.Error.userError : String → IO.Error

EIO ε α represents IO actions that will either terminate with an error of type ε or succeed with a value of type α. This means that, like the Except ε monad, the IO monad includes the ability to define error handling and exceptions.

Peeling back another layer, EIO is itself defined in terms of a simpler structure:

#print EIO
def EIO : Type → Type → Type :=
fun ε => EStateM ε IO.RealWorld

The EStateM monad includes both errors and state—it's a combination of Except and State. It is defined using another type, EStateM.Result:

#print EStateM
def EStateM.{u} : Type u → Type u → Type u → Type u :=
fun ε σ α => σ → EStateM.Result ε σ α

In other words, a program with type EStateM ε σ α is a function that accepts an initial state of type σ and returns an EStateM.Result ε σ α.

EStateM.Result is very much like the definition of Except, with one constructor that indicates a successful termination and one constructor that indicates an error:

#print EStateM.Result
inductive EStateM.Result.{u} : Type u → Type u → Type u → Type u
number of parameters: 3
EStateM.Result.ok : {ε σ α : Type u} → α → σ → EStateM.Result ε σ α
EStateM.Result.error : {ε σ α : Type u} → ε → σ → EStateM.Result ε σ α

Just like Except ε α, the ok constructor includes a result of type α, and the error constructor includes an exception of type ε. Unlike Except, both constructors have an additional state field that includes the final state of the computation.

The Monad instance for EStateM ε σ requires pure and bind. Just as with State, the implementation of pure for EStateM accepts an initial state and returns it unchanged, and just as with Except, it returns its argument in the ok constructor:

#print EStateM.pure
protected def EStateM.pure.{u} : {ε σ α : Type u} → α → EStateM ε σ α :=
fun {ε σ α} a s => EStateM.Result.ok a s

protected means that the full name EStateM.pure is needed even if the EStateM namespace has been opened.

Similarly, bind for EStateM takes an initial state as an argument. It passes this initial state to its first action. Like bind for Except, it then checks whether the result is an error. If so, the error is returned unchanged and the second argument to bind remains unused. If the result was a success, then the second argument is applied to both the returned value and to the resulting state.

#print EStateM.bind
protected def EStateM.bind.{u} : {ε σ α β : Type u} → EStateM ε σ α → (α → EStateM ε σ β) → EStateM ε σ β :=
fun {ε σ α β} x f s =>
  match x s with
  | EStateM.Result.ok a s => f a s
  | EStateM.Result.error e s => EStateM.Result.error e s

Putting all of this together, IO is a monad that tracks state and errors at the same time. The collection of available errors is that given by the datatype IO.Error, which has constructors that describe many things that can go wrong in a program. The state is a type that represents the real world, called IO.RealWorld. Each basic IO action receives this real world and returns another one, paired either with an error or a result. In IO, pure returns the world unchanged, while bind passes the modified world from one action into the next action.

Because the entire universe doesn't fit in a computer's memory, the world being passed around is just a representation. So long as world tokens are not re-used, the representation is safe. This means that world tokens do not need to contain any data at all:

#print IO.RealWorld
def IO.RealWorld : Type :=