ReaderT is far from the only useful monad transformer. This section describes a number of additional transformers. Each monad transformer consists of the following:

1. A definition or datatype T that takes a monad as an argument. It should have a type like (Type u → Type v) → Type u → Type v, though it may accept additional arguments prior to the monad.
2. A Monad instance for T m that relies on an instance of Monad m. This enables the transformed monad to be used as a monad.
3. A MonadLift instance that translates actions of type m α into actions of type T m α, for arbitrary monads m. This enables actions from the underlying monad to be used in the transformed monad.

Furthermore, the Monad instance for the transformer should obey the contract for Monad, at least if the underlying Monad instance does. In addition, monadLift (pure x) should be equivalent to pure x in the transformed monad, and monadLift should distribute over bind so that monadLift (x >>= f) is the same as monadLift x >>= fun y => monadLift (f y).

Many monad transformers additionally define type classes in the style of MonadReader that describe the actual effects available in the monad. This can provide more flexibility: it allows programs to be written that rely only on an interface, and don't constrain the underlying monad to be implemented by a given transformer. The type classes are a way for programs to express their requirements, and monad transformers are a convenient way to meet these requirements.

## Failure with OptionT

Failure, represented by the Option monad, and exceptions, represented by the Except monad, both have corresponding transformers. In the case of Option, failure can be added to a monad by having it contain values of type Option α where it would otherwise contain values of type α. For example, IO (Option α) represents IO actions that don't always return a value of type α. This suggests the definition of the monad transformer OptionT:

def OptionT (m : Type u → Type v) (α : Type u) : Type v :=
m (Option α)


An example of OptionT in action, consider a program that asks the user questions. The function getSomeInput asks for a line of input and removes whitespace from both ends. If the resulting trimmed input is non-empty, then it is returned, but the function fails if there are no non-whitespace characters:

def getSomeInput : OptionT IO String := do
let input ← (← IO.getStdin).getLine
let trimmed := input.trim
if trimmed == "" then
failure
else pure trimmed


This particular application tracks users with their name and their favorite species of beetle:

structure UserInfo where
name : String
favoriteBeetle : String


Asking the user for input is no more verbose than a function that uses only IO would be:

def getUserInfo : OptionT IO UserInfo := do
let name ← getSomeInput
IO.println "What is your favorite species of beetle?"
let beetle ← getSomeInput
pure ⟨name, beetle⟩


However, because the function runs in an OptionT IO context rather than just in IO, failure in the first call to getSomeInput causes the whole getUserInfo to fail, with control never reaching the question about beetles. The main function, interact, invokes getUserInfo in a purely IO context, which allows it to check whether the call succeeded or failed by matching on the inner Option:

def interact : IO Unit := do
match ← getUserInfo with
| none => IO.eprintln "Missing info"
| some ⟨name, beetle⟩ => IO.println s!"Hello {name}, whose favorite beetle is {beetle}."


Writing the monad instance reveals a difficulty. Based on the types, pure should use pure from the underlying monad m together with some. Just as bind for Option branches on the first argument, propagating none, bind for OptionT should run the monadic action that makes up the first argument, branch on the result, and then propagate none. Following this sketch yields the following definition, which Lean does not accept:

instance [Monad m] : Monad (OptionT m) where
pure x := pure (some x)
bind action next := do
match (← action) with
| none => pure none
| some v => next v


The error message shows a cryptic type mismatch:

application type mismatch
pure (some x)
argument
some x
has type
Option α✝ : Type ?u.28
but is expected to have type
α✝ : Type ?u.28


The problem here is that Lean is selecting the wrong Monad instance for the surrounding use of pure. Similar errors occur for the definition of bind. One solution is to use type annotations to guide Lean to the correct Monad instance:

instance [Monad m] : Monad (OptionT m) where
pure x := (pure (some x) : m (Option _))
bind action next := (do
match (← action) with
| none => pure none
| some v => next v : m (Option _))


While this solution works, it is inelegant and the code becomes a bit noisy.

An alternative solution is to define functions whose type signatures guide Lean to the correct instances. In fact, OptionT could have been defined as a structure:

structure OptionT (m : Type u → Type v) (α : Type u) : Type v where
run : m (Option α)


This would solve the problem, because the constructor OptionT.mk and the field accessor OptionT.run would guide type class inference to the correct instances. The downside to doing this is that structure values would need to be allocated and deallocated repeatedly when running code that uses it, while the direct definition is a compile-time-only feature. The best of both worlds can be achieved by defining functions that serve the same role as OptionT.mk and OptionT.run, but that work with the direct definition:

def OptionT.mk (x : m (Option α)) : OptionT m α := x

def OptionT.run (x : OptionT m α) : m (Option α) := x


Both functions return their inputs unchanged, but they indicate the boundary between code that is intended to present the interface of OptionT and code that is intended to present the interface of the underlying monad m. Using these helpers, the Monad instance becomes more readable:

instance [Monad m] : Monad (OptionT m) where
pure x := OptionT.mk (pure (some x))
bind action next := OptionT.mk do
match ← action with
| none => pure none
| some v => next v


Here, the use of OptionT.mk indicates that its arguments should be considered as code that uses the interface of m, which allows Lean to select the correct Monad instances.

After defining the monad instance, it's a good idea to check that the monad contract is satisfied. The first step is to show that bind (pure v) f is the same as f v. Here's the steps:

bind (pure v) f
={ Unfolding the definitions of bind and pure }=
OptionT.mk do
match ← pure (some v) with
| none => pure none
| some x => f x
={ Desugaring nested action syntax }=
OptionT.mk do
let y ← pure (some v)
match y with
| none => pure none
| some x => f x
={ Desugaring do-notation }=
OptionT.mk
(pure (some v) >>= fun y =>
match y with
| none => pure none
| some x => f x)
={ Using the first monad rule for m }=
OptionT.mk
(match some v with
| none => pure none
| some x => f x)
={ Reduce match }=
OptionT.mk (f v)
={ Definition of OptionT.mk }=
f v

The second rules states that bind w pure is the same as w. To demonstrate this, unfold the definitions of bind and pure, yielding:

OptionT.mk do
match ← w with
| none => pure none
| some v => pure (some v)


In this pattern match, the result of both cases is the same as the pattern being matched, just with pure around it. In other words, it is equivalent to w >>= fun y => pure y, which is an instance of m's second monad rule.

The final rule states that bind (bind v f) g is the same as bind v (fun x => bind (f x) g). It can be checked in the same way, by expanding the definitions of bind and pure and then delegating to the underlying monad m.

### An Alternative Instance

One convenient way to use OptionT is through the Alternative type class. Successful return is already indicated by pure, and the failure and orElse methods of Alternative provide a way to write a program that returns the first successful result from a number of subprograms:

instance [Monad m] : Alternative (OptionT m) where
failure := OptionT.mk (pure none)
orElse x y := OptionT.mk do
match ← x with
| some result => pure (some result)
| none => y ()


### Lifting

Lifting an action from m to OptionT m only requires wrapping some around the result of the computation:

instance [Monad m] : MonadLift m (OptionT m) where
pure (some (← action))


## Exceptions

The monad transformer version of Except is very similar to the monad transformer version of Option. Adding exceptions of type ε to some monadic action of type m α can be accomplished by adding exceptions to α, yielding type m (Except ε α):

def ExceptT (ε : Type u) (m : Type u → Type v) (α : Type u) : Type v :=
m (Except ε α)


OptionT provides mk and run functions to guide the type checker towards the correct Monad instances. This trick is also useful for ExceptT:

def ExceptT.mk {ε α : Type u} (x : m (Except ε α)) : ExceptT ε m α := x

def ExceptT.run {ε α : Type u} (x : ExceptT ε m α) : m (Except ε α) := x


The Monad instance for ExceptT is also very similar to the instance for OptionT. The only difference is that it propagates a specific error value, rather than none:

instance {ε : Type u} {m : Type u → Type v} [Monad m] : Monad (ExceptT ε m) where
pure x := ExceptT.mk (pure (Except.ok x))
bind result next := ExceptT.mk do
match ← result with
| .error e => pure (.error e)
| .ok x => next x


The type signatures of ExceptT.mk and ExceptT.run contain a subtle detail: they annotate the universe levels of α and ε explicitly. If they are not explicitly annotated, then Lean generates a more general type signature in which they have distinct polymorphic universe variables. However, the definition of ExceptT expects them to be in the same universe, because they can both be provided as arguments to m. This can lead to a problem in the Monad instance where the universe level solver fails to find a working solution:

def ExceptT.mk (x : m (Except ε α)) : ExceptT ε m α := x

instance {ε : Type u} {m : Type u → Type v} [Monad m] : Monad (ExceptT ε m) where
pure x := ExceptT.mk (pure (Except.ok x))
bind result next := ExceptT.mk do
match (← result) with
| .error e => pure (.error e)
| .ok x => next x

stuck at solving universe constraint
max ?u.12268 ?u.12269 =?= u
while trying to unify
ExceptT ε m α✝
with
(ExceptT ε m α✝) ε m α✝


This kind of error message is typically caused by underconstrained universe variables. Diagnosing it can be tricky, but a good first step is to look for reused universe variables in some definitions that are not reused in others.

Unlike Option, the Except datatype is typically not used as a data structure. It is always used as a control structure with its Monad instance. This means that it is reasonable to lift Except ε actions into ExceptT ε m, as well as actions from the underlying monad m. Lifting Except actions into ExceptT actions is done by wrapping them in m's pure, because an action that only has exception effects cannot have any effects from the monad m:

instance [Monad m] : MonadLift (Except ε) (ExceptT ε m) where
monadLift action := ExceptT.mk (pure action)


Because actions from m do not have any exceptions in them, their value should be wrapped in Except.ok. This can be accomplished using the fact that Functor is a superclass of Monad, so applying a function to the result of any monadic computation can be accomplished using Functor.map:

instance [Monad m] : MonadLift m (ExceptT ε m) where
monadLift action := ExceptT.mk (.ok <\$> action)


### Type Classes for Exceptions

Exception handling fundamentally consists of two operations: the ability to throw exceptions, and the ability to recover from them. Thus far, this has been accomplished using the constructors of Except and pattern matching, respectively. However, this ties a program that uses exceptions to one specific encoding of the exception handling effect. Using a type class to capture these operations allows a program that uses exceptions to be used in any monad that supports throwing and catching.

Throwing an exception should take an exception as an argument, and it should be allowed in any context where a monadic action is requested. The "any context" part of the specification can be written as a type by writing m α—because there's no way to produce a value of any arbitrary type, the throw operation must be doing something that causes control to leave that part of the program. Catching an exception should accept any monadic action together with a handler, and the handler should explain how to get back to the action's type from an exception:

class MonadExcept (ε : outParam (Type u)) (m : Type v → Type w) where
throw : ε → m α
tryCatch : m α → (ε → m α) → m α


The universe levels on MonadExcept differ from those of ExceptT. In ExceptT, both ε and α have the same level, while MonadExcept imposes no such limitation. This is because MonadExcept never places an exception value inside of m. The most general universe signature recognizes the fact that ε and α are completely independent in this definition. Being more general means that the type class can be instantiated for a wider variety of types.

An example program that uses MonadExcept is a simple division service. The program is divided into two parts: a frontend that supplies a user interface based on strings that handles errors, and a backend that actually does the division. Both the frontend and the backend can throw exceptions, the former for ill-formed input and the latter for division by zero errors. The exceptions are an inductive type:

inductive Err where
| divByZero
| notANumber : String → Err


The backend checks for zero, and divides if it can:

def divBackend [Monad m] [MonadExcept Err m] (n k : Int) : m Int :=
if k == 0 then
throw .divByZero
else pure (n / k)


The frontend's helper asNumber throws an exception if the string it is passed is not a number. The overall frontend converts its inputs to Ints and calls the backend, handling exceptions by returning a friendly string error:

def asNumber [Monad m] [MonadExcept Err m] (s : String) : m Int :=
match s.toInt? with
| none => throw (.notANumber s)
| some i => pure i

def divFrontend [Monad m] [MonadExcept Err m] (n k : String) : m String :=
tryCatch (do pure (toString (← divBackend (← asNumber n) (← asNumber k))))
fun
| .divByZero => pure "Division by zero!"
| .notANumber s => pure s!"Not a number: \"{s}\""


Throwing and catching exceptions is common enough that Lean provides a special syntax for using MonadExcept. Just as + is short for HAdd.hAdd, try and catch can be used as shorthand for the tryCatch method:

def divFrontend [Monad m] [MonadExcept Err m] (n k : String) : m String :=
try
pure (toString (← divBackend (← asNumber n) (← asNumber k)))
catch
| .divByZero => pure "Division by zero!"
| .notANumber s => pure s!"Not a number: \"{s}\""


In addition to Except and ExceptT, there are useful MonadExcept instances for other types that may not seem like exceptions at first glance. For example, failure due to Option can be seen as throwing an exception that contains no data whatsoever, so there is an instance of MonadExcept Unit Option that allows try ... catch ... syntax to be used with Option.

## State

A simulation of mutable state is added to a monad by having monadic actions accept a starting state as an argument and return a final state together with their result. The bind operator for a state monad provides the final state of one action as an argument to the next action, threading the state through the program. This pattern can also be expressed as a monad transformer:

def StateT (σ : Type u) (m : Type u → Type v) (α : Type u) : Type (max u v) :=
σ → m (α × σ)


Once again, the monad instance is very similar to that for State. The only difference is that the input and output states are passed around and returned in the underlying monad, rather than with pure code:

instance [Monad m] : Monad (StateT σ m) where
pure x := fun s => pure (x, s)
bind result next := fun s => do
let (v, s') ← result s
next v s'


The corresponding type class has get and set methods. One downside of get and set is that it becomes too easy to set the wrong state when updating it. This is because retrieving the state, updating it, and saving the updated state is a natural way to write some programs. For example, the following program counts the number of diacritic-free English vowels and consonants in a string of letters:

structure LetterCounts where
vowels : Nat
consonants : Nat
deriving Repr

inductive Err where
| notALetter : Char → Err
deriving Repr

def vowels :=
let lowerVowels := "aeiuoy"
lowerVowels ++ lowerVowels.map (·.toUpper)

def consonants :=
let lowerConsonants := "bcdfghjklmnpqrstvwxz"
lowerConsonants ++ lowerConsonants.map (·.toUpper )

def countLetters (str : String) : StateT LetterCounts (Except Err) Unit :=
let rec loop (chars : List Char) := do
match chars with
| [] => pure ()
| c :: cs =>
let st ← get
let st' ←
if c.isAlpha then
if vowels.contains c then
pure {st with vowels := st.vowels + 1}
else if consonants.contains c then
pure {st with consonants := st.consonants + 1}
else -- modified or non-English letter
pure st
else throw (.notALetter c)
set st'
loop cs
loop str.toList


It would be very easy to write set st instead of set st'. In a large program, this kind of mistake can lead to difficult-to-diagnose bugs.

While using a nested action for the call to get would solve this problem, it can't solve all such problems. For example, a function might update a field on a structure based on the values of two other fields. This would require two separate nested-action calls to get. Because the Lean compiler contains optimizations that are only effective when there is a single reference to a value, duplicating the references to the state might lead to code that is significantly slower. Both the potential performance problem and the potential bug can be worked around by using modify, which transforms the state using a function:

def countLetters (str : String) : StateT LetterCounts (Except Err) Unit :=
let rec loop (chars : List Char) := do
match chars with
| [] => pure ()
| c :: cs =>
if c.isAlpha then
if vowels.contains c then
modify fun st => {st with vowels := st.vowels + 1}
else if consonants.contains c then
modify fun st => {st with consonants := st.consonants + 1}
else -- modified or non-English letter
pure ()
else throw (.notALetter c)
loop cs
loop str.toList


The type class contains a function akin to modify called modifyGet, which allows the function to both compute a return value and transform an old state in a single step. The function returns a pair in which the first element is the return value, and the second element is the new state; modify just adds the constructor of Unit to the pair used in modifyGet:

def modify [MonadState σ m] (f : σ → σ) : m Unit :=
modifyGet fun s => ((), f s)


The definition of MonadState is as follows:

class MonadState (σ : outParam (Type u)) (m : Type u → Type v) : Type (max (u+1) v) where
get : m σ
set : σ → m PUnit
modifyGet : (σ → α × σ) → m α


PUnit is a version of the Unit type that is universe-polymorphic to allow it to be in Type u instead of Type. While it would be possible to provide a default implementation of modifyGet in terms of get and set, it would not admit the optimizations that make modifyGet useful in the first place, rendering the method useless.

## Of Classes and The Functions

Thus far, each monad type class that takes extra information, like the type of exceptions for MonadExcept or the type of the state for MonadState, has this type of extra information as an output parameter. For simple programs, this is generally convenient, because a monad that combines one use each of StateT, ReaderT, and ExceptT has only a single state type, environment type, and exception type. As monads grow in complexity, however, they may involve multiple states or errors types. In this case, the use of an output parameter makes it impossible to target both states in the same do-block.

For these cases, there are additional type classes in which the extra information is not an output parameter. These versions of the type classes use the word Of in the name. For example, MonadStateOf is like MonadState, but without an outParam modifier.

Similarly, there are versions of the type class methods that accept the type of the extra information as an explicit, rather than implicit, argument. For MonadStateOf, there are getThe with type

(σ : Type u) → {m : Type u → Type v} → [MonadStateOf σ m] → m σ


and modifyThe with type

(σ : Type u) → {m : Type u → Type v} → [MonadStateOf σ m] → (σ → σ) → m PUnit


There is no setThe because the type of the new state is enough to decide which surrounding state monad transformer to use.

In the Lean standard library, there are instances of the non-Of versions of the classes defined in terms of the instances of the versions with Of. In other words, implementing the Of version yields implementations of both. It's generally a good idea to implement the Of version, and then start writing programs using the non-Of versions of the class, transitioning to the Of version if the output parameter becomes inconvenient.

## Transformers and Id

The identity monad Id is the monad that has no effects whatsoever, to be used in contexts that expect a monad for some reason but where none is actually necessary. Another use of Id is to serve as the bottom of a stack of monad transformers. For instance, StateT σ Id works just like State σ.

## Exercises

Define a monad transformer version of WithLog. Also define the corresponding type class MonadWithLog, and write a program that combines logging and exceptions.
Modify doug's monad with StateT such that it counts the number of directories and files seen. At the end of execution, it should display a report like:
  Viewed 38 files in 5 directories.