# Note to self: reader monad transformer

Note to self on constructing a monad transformer. In a way this follows on from the earlier post Applicatives compose, monads do not. Literate Haskell source for this post is available here: https://github.com/carlohamalainen/playground/tree/master/haskell/transformers.

## The Reader Monad

A `Reader`

is a data type that encapsulates an environment. The `runReader`

function takes an environment (a `Reader`

) and runs it, producing the result of type `a`

.

Note that with record syntax, the type of `runReader`

is actually

Reader that increments its environment value, returning the same type:

Reader that converts its environment value into a string:

“Run” the reader using `runReader`

:

```
ghci> runReader reader1 100
101
ghci> runReader reader2 100
"reader2: 100"
```

There’s nothing magic about `runReader`

; it is just taking the function out
of the data type. We can do it manually ourselves:

```
ghci> runReader' reader1 100
101
```

Next, make `Reader`

an instance of `Monad`

:

The definition of `>>=`

is relatively easy to work out using hole driven development.

Example usage:

Or in the more readable `do`

notation:

```
ghci> runReader eg1' 100
"hey, 100"
```

Note that we use `id`

to produce a `Reader`

that just passes its environment argument along as the output. See `readerAsk`

later for the equivalent situation which uses `return`

for `MonadReader`

.

Since `[]`

is an instance of `Monad`

we can also do things like this:

```
ghci> runReader eg1'' 100
"hey, [100]"
```

## Reader Transformer

We’d like to use the `Reader`

in conjunction with other monads, for example running `IO`

actions but
having access to the reader’s environment. To do this we create a transformer, which we’ll call `ReaderT`

:

The `r`

parameter is the reader environment as before, `m`

is the monad (for example `IO`

), and `a`

is the result type, as before. Again, note the actual type of `runReaderT`

:

It takes a `ReaderT`

and provides us with a function that takes a reader environment of type `r`

and produces a monadic value. Following from the `Monad`

instance declaration for `Reader`

it is straightforward to write the definition for `ReaderT`

.

With a `ReaderT`

as the outer monad, we would like to “lift” a monadic computation into the reader. A monadic action doesn’t know anything about the reader environment, so to lift the monadic value we just create a `ReaderT`

with a function that ignores the environment:

Example usage:

Note the type of `return`

on the first line of `egLift`

. In this context, `return :: a -> m a`

is the equivalent of `id :: a -> a`

from the earlier `Reader`

example.

```
ghci> runReaderT egLift 100
"boo"
"value of e: 100"
```

More generally, let’s name the `ask`

function:

If we want to modify the environment, we use `withReaderT`

which takes as its first parameter a function to modify the environment. Note that the result is of type `ReaderT r' m a`

so the function is of type `r' -> r`

which modifies the supplied reader of type `ReaderT r m a`

.

Lastly, it is convenient to apply a function to the current environment.

This is almost the same as `readerAsk`

except that we create a reader that returns `f r`

instead of `f`

. In other words:

Finally, we collect the functions `readerAsk`

, `withReader`

, and `readerReader`

in a type class `MonadReader`

and give them more general names:

An instance declaration for our `ReaderT`

type:

Now we can write fairly succinct code as follows. Use the `IO`

monad as the inner monad in a `ReaderT`

, with an `Int`

environment and `String`

result type.

```
ghci> result <- runReaderT eg2 100
"I'm in the eg2 function and the environment is: 100"
ghci> result
"returned value: 100"
```

All of the above is available from Control.Monad.Reader and Control.Monad.Trans.

## StateT, ReaderT, and ask

The State monad encapsulates a modifiable state. It has a transformer `StateT`

as one would expect. Yet we are able to call `ask`

inside a `StateT`

monad. Here is an example (the raw code is here):

The function `inside0`

has the `IO`

monad nested inside a `Reader`

, while `inside1`

has a `StateT`

with the `ReaderT`

inside. Yet in both we can write `e <- ask`

.

Inspecting the types using ghcmod-vim we find that

so there must be a type class that provides the `ask`

function for `StateT`

.

First, inspect the type of `ask`

using ghci (here we are using the definitions from `Control.Monad.Reader`

and `Control.Monad.State`

, not the implementation in this file).

```
ghci> :t ask
ask :: MonadReader r m => m r
```

So `StateT`

must have a `MonadReader`

instance. Confirm this with `:i`

:

```
ghci> :i StateT
(lots of stuff)
instance MonadReader r m => MonadReader r (StateT s m)
-- Defined in `Control.Monad.Reader.Class'
(lots of stuff)
```

Looking in Control.Monad.Reader.Class we find:

The `lift`

function comes from Monad.Trans.Class,
and looking there we see:

So actually we are after the `MonaTrans`

type class. Again looking at `:i`

on `StateT`

we see:

```
ghci> :i StateT
instance MonadTrans (StateT s)
(lots of stuff)
-- Defined in `Control.Monad.Trans.State.Lazy'
(lots of stuff)
```

So off we go to Control.Monad.Trans.State.Lazy where we finally get the answer:

This shows that for `StateT`

, the `lift`

function takes a monadic action and produces a state transformer that takes the current state, runs the action, and returns the result of the action along with the unmodified state. This makes sense in that the underlying action should not modify the state. (There are some laws that monad transformers must satisfy.)

If we did not have the `MonadTrans`

type class then we would have to embed the `ask`

call manually:

Obviously this is laborious and error-prone. In this case, Haskell’s type class system lets us implement a few classes so that `ask`

, `get`

, `put`

, etc, can be used seamlessly no matter which monad transformer we are in.

The downside is that reading Haskell code can be nontrivial. In our case we had to follow a trail through a few files to see where `ask`

was actually implemented, and finding the right definition relied on us being able to infer the correct types of certain sub-expressions.

Personally I am finding more and more that plain vim and ghci does not cut it for Haskell development, and something richer like ghcmod-vim is a real necessity. Shameless self plug: ghc-imported-from is also very useful :-)