# Simple Effects¶

So far we have seen how to write programs with locally mutable state
using the `STATE`

effect. To recap, we have the definitions below
in a module `Effect.State`

```
module Effect.State
STATE : Type -> EFFECT
get : Eff x [STATE x]
put : x -> Eff () [STATE x]
putM : y -> Eff () [STATE x] [STATE y]
update : (x -> x) -> Eff () [STATE x]
instance Handler State m
```

The last line, `instance Handler State m`

, means that the `STATE`

effect is usable in any computation context `m`

. That is, a program
which uses this effect and returns something of type `a`

can be
evaluated to something of type `m a`

using `run`

, for any
`m`

. The lower case `State`

is a data type describing the
operations which make up the `STATE`

effect itself—we will go into
more detail about this in Section Creating New Effects.

In this section, we will introduce some other supported effects,
allowing console I/O, exceptions, random number generation and
non-deterministic programming. For each effect we introduce, we will
begin with a summary of the effect, its supported operations, and the
contexts in which it may be used, like that above for `STATE`

, and
go on to present some simple examples. At the end, we will see some
examples of programs which combine multiple effects.

All of the effects in the library, including those described in this section, are summarised in Appendix Effects Summary.

## Console I/O¶

Console I/O is supported with the `STDIO`

effect, which allows reading and writing characters and strings to and
from standard input and standard output. Notice that there is a
constraint here on the computation context `m`

, because it only
makes sense to support console I/O operations in a context where we
can perform (or at the very least simulate) console I/O:

```
module Effect.StdIO
STDIO : EFFECT
putChar : Char -> Eff () [STDIO]
putStr : String -> Eff () [STDIO]
putStrLn : String -> Eff () [STDIO]
getStr : Eff String [STDIO]
getChar : Eff Char [STDIO]
instance Handler StdIO IO
instance Handler StdIO (IOExcept a)
```

### Examples¶

A program which reads the user’s name, then says hello, can be written as follows:

```
hello : Eff () [STDIO]
hello = do putStr "Name? "
x <- getStr
putStrLn ("Hello " ++ trim x ++ "!")
```

We use `trim`

here to remove the trailing newline from the
input. The resource associated with `STDIO`

is simply the empty
tuple, which has a default value `()`

, so we can run this as
follows:

```
main : IO ()
main = run hello
```

In `hello`

we could also use `!`

-notation instead of ```
x <-
getStr
```

, since we only use the string that is read once:

```
hello : Eff () [STDIO]
hello = do putStr "Name? "
putStrLn ("Hello " ++ trim !getStr ++ "!")
```

More interestingly, we can combine multiple effects in one program. For example, we can loop, counting the number of people we’ve said hello to:

```
hello : Eff () [STATE Int, STDIO]
hello = do putStr "Name? "
putStrLn ("Hello " ++ trim !getStr ++ "!")
update (+1)
putStrLn ("I've said hello to: " ++ show !get ++ " people")
hello
```

The list of effects given in `hello`

means that the function can
call `get`

and `put`

on an integer state, and any functions which
read and write from the console. To run this, `main`

does not need
to be changed.

### Aside: Resource Types¶

To find out the resource type of an effect, if necessary (for example
if we want to initialise a resource explicitiy with `runInit`

rather
than using a default value with `run`

) we can run the
`resourceType`

function at the REPL:

```
*ConsoleIO> resourceType STDIO
() : Type
*ConsoleIO> resourceType (STATE Int)
Int : Type
```

## Exceptions¶

The `EXCEPTION`

effect is declared in module `Effect.Exception`

. This allows programs
to exit immediately with an error, or errors to be handled more
generally:

```
module Effect.Exception
EXCEPTION : Type -> EFFECT
raise : a -> Eff b [EXCEPTION a]
instance Handler (Exception a) Maybe
instance Handler (Exception a) List
instance Handler (Exception a) (Either a)
instance Handler (Exception a) (IOExcept a)
instance Show a => Handler (Exception a) IO
```

### Example¶

Suppose we have a `String`

which is expected to represent an integer
in the range `0`

to `n`

. We can write a function `parseNumber`

which returns an `Int`

if parsing the string returns a number in the
appropriate range, or throws an exception otherwise. Exceptions are
paramaterised by an error type:

```
data Err = NotANumber | OutOfRange
parseNumber : Int -> String -> Eff Int [EXCEPTION Err]
parseNumber num str
= if all isDigit (unpack str)
then let x = cast str in
if (x >=0 && x <= num)
then pure x
else raise OutOfRange
else raise NotANumber
```

Programs which support the `EXCEPTION`

effect can be run in any
context which has some way of throwing errors, for example, we can run
`parseNumber`

in the `Either Err`

context. It returns a value of
the form `Right x`

if successful:

```
*Exception> the (Either Err Int) $ run (parseNumber 42 "20")
Right 20 : Either Err Int
```

Or `Left e`

on failure, carrying the appropriate exception:

```
*Exception> the (Either Err Int) $ run (parseNumber 42 "50")
Left OutOfRange : Either Err Int
*Exception> the (Either Err Int) $ run (parseNumber 42 "twenty")
Left NotANumber : Either Err Int
```

In fact, we can do a little bit better with `parseNumber`

, and have
it return a *proof* that the integer is in the required range along
with the integer itself. One way to do this is define a type of
bounded integers, `Bounded`

:

```
Bounded : Int -> Type
Bounded x = (n : Int ** So (n >= 0 && n <= x))
```

Recall that `So`

is parameterised by a `Bool`

, and only ```
So
True
```

is inhabited. We can use `choose`

to construct such a value
from the result of a dynamic check:

```
data So : Bool -> Type = Oh : So True
choose : (b : Bool) -> Either (So b) (So (not b))
```

We then write `parseNumber`

using `choose`

rather than an
`if/then/else`

construct, passing the proof it returns on success as
the boundedness proof:

```
parseNumber : (x : Int) -> String -> Eff (Bounded x) [EXCEPTION Err]
parseNumber x str
= if all isDigit (unpack str)
then let num = cast str in
case choose (num >=0 && num <= x) of
Left p => pure (num ** p)
Right p => raise OutOfRange
else raise NotANumber
```

## Random Numbers¶

Random number generation is also implemented by the library, in module
`Effect.Random`

:

```
module Effect.Random
RND : EFFECT
srand : Integer -> Eff () [RND]
rndInt : Integer -> Integer -> Eff Integer [RND]
rndFin : (k : Nat) -> Eff (Fin (S k)) [RND]
instance Handler Random m
```

Random number generation is considered side-effecting because its
implementation generally relies on some external source of randomness.
The default implementation here relies on an integer *seed*, which can
be set with `srand`

. A specific seed will lead to a predictable,
repeatable sequence of random numbers. There are two functions which
produce a random number:

`rndInt`

, which returns a random integer between the given lowerand upper bounds.

`rndFin`

, which returns a random element of a finite set(essentially a number with an upper bound given in its type).

### Example¶

We can use the `RND`

effect to implement a simple guessing game. The
`guess`

function, given a target number, will repeatedly ask the
user for a guess, and state whether the guess is too high, too low, or
correct:

```
guess : Int -> Eff () [STDIO]
```

For reference, the code for `guess`

is given below:

```
guess : Int -> Eff () [STDIO]
guess target
= do putStr "Guess: "
case run {m=Maybe} (parseNumber 100 (trim !getStr)) of
Nothing => do putStrLn "Invalid input"
guess target
Just (v ** _) =>
case compare v target of
LT => do putStrLn "Too low"
guess target
EQ => putStrLn "You win!"
GT => do putStrLn "Too high"
guess target
```

Note that we use `parseNumber`

as defined previously to read user input, but
we don’t need to list the `EXCEPTION`

effect because we use a nested `run`

to invoke `parseNumber`

, independently of the calling effectful program.

To invoke this, we pick a random number within the range 0–100,
having set up the random number generator with a seed, then run
`guess`

:

```
game : Eff () [RND, STDIO]
game = do srand 123456789
guess (fromInteger !(rndInt 0 100))
main : IO ()
main = run game
```

If no seed is given, it is set to the `default`

value. For a less
predictable game, some better source of randomness would be required,
for example taking an initial seed from the system time. To see how to
do this, see the `SYSTEM`

effect described in Effects Summary.

## Non-determinism¶

The listing below gives the definition of the non-determinism effect, which allows a program to choose a value non-deterministically from a list of possibilities in such a way that the entire computation succeeds:

```
import Effects
import Effect.Select
SELECT : EFFECT
select : List a -> Eff a [SELECT]
instance Handler Selection Maybe
instance Handler Selection List
```

### Example¶

The `SELECT`

effect can be used to solve constraint problems, such
as finding Pythagorean triples. The idea is to use `select`

to give
a set of candidate values, then throw an exception for any combination
of values which does not satisfy the constraint:

```
triple : Int -> Eff (Int, Int, Int) [SELECT, EXCEPTION String]
triple max = do z <- select [1..max]
y <- select [1..z]
x <- select [1..y]
if (x * x + y * y == z * z)
then pure (x, y, z)
else raise "No triple"
```

This program chooses a value for `z`

between `1`

and `max`

, then
values for `y`

and `x`

. In operation, after a `select`

, the
program executes the rest of the `do`

-block for every possible
assignment, effectively searching depth-first. If the list is empty
(or an exception is thrown) execution fails.

There are handlers defined for `Maybe`

and `List`

contexts, i.e.
contexts which can capture failure. Depending on the context `m`

,
`triple`

will either return the first triple it finds (if in
`Maybe`

context) or all triples in the range (if in `List`

context). We can try this as follows:

```
main : IO ()
main = do print $ the (Maybe _) $ run (triple 100)
print $ the (List _) $ run (triple 100)
```

`vadd`

revisited¶

We now return to the `vadd`

program from the introduction. Recall the
definition:

```
vadd : Vect n Int -> Vect n Int -> Vect n Int
vadd [] [] = []
vadd (x :: idris xs) (y :: ys) = x + y :: vadd xs ys
```

Using , we can set up a program so that it reads input from a user,
checks that the input is valid (i.e both vectors contain integers, and
are the same length) and if so, pass it on to `vadd`

. First, we
write a wrapper for `vadd`

which checks the lengths and throw an
exception if they are not equal. We can do this for input vectors of
length `n`

and `m`

by matching on the implicit arguments `n`

and
`m`

and using `decEq`

to produce a proof of their equality, if
they are equal:

```
vadd_check : Vect n Int -> Vect m Int ->
Eff (Vect m Int) [EXCEPTION String]
vadd_check {n} {m} xs ys with (decEq n m)
vadd_check {n} {m=n} xs ys | (Yes Refl) = pure (vadd xs ys)
vadd_check {n} {m} xs ys | (No contra) = raise "Length mismatch"
```

To read a vector from the console, we implement a function of the following type:

```
read_vec : Eff (p ** Vect p Int) [STDIO]
```

This returns a dependent pair of a length, and a vector of that
length, because we cannot know in advance how many integers the user
is going to input. One way to implement this function, using `-1`

to
indicate the end of input, is shown in Listing [readvec]. This uses a
variation on `parseNumber`

which does not require a number to be
within range.

Finally, we write a program which reads two vectors and prints the result of pairwise addition of them, throwing an exception if the inputs are of differing lengths:

```
do_vadd : Eff () [STDIO, EXCEPTION String]
do_vadd = do putStrLn "Vector 1"
(_ ** xs) <- read_vec
putStrLn "Vector 2"
(_ ** ys) <- read_vec
putStrLn (show !(vadd_check xs ys))
```

By having explicit lengths in the type, we can be sure that `vadd`

is only being used where the lengths of inputs are guaranteed to be
equal. This does not stop us reading vectors from user input, but it
does require that the lengths are checked and any discrepancy is dealt
with gracefully.

```
read_vec : Eff (p ** Vect p Int) [STDIO]
read_vec = do putStr "Number (-1 when done): "
case run (parseNumber (trim !getStr)) of
Nothing => do putStrLn "Input error"
read_vec
Just v => if (v /= -1)
then do (_ ** xs) <- read_vec
pure (_ ** v :: xs)
else pure (_ ** [])
where
parseNumber : String -> Eff Int [EXCEPTION String]
parseNumber str
= if all (\x => isDigit x || x == '-') (unpack str)
then pure (cast str)
else raise "Not a number"
```

## Example: An Expression Calculator¶

To show how these effects can fit together, let us consider an evaluator for a simple expression language, with addition and integer values.

```
data Expr = Val Integer
| Add Expr Expr
```

An evaluator for this language always returns an `Integer`

, and
there are no situations in which it can fail!

```
eval : Expr -> Integer
eval (Val x) = x
eval (Add l r) = eval l + eval r
```

If we add variables, however, things get more interesting. The evaluator will need to be able to access the values stored in variables, and variables may be undefined.

```
data Expr = Val Integer
| Var String
| Add Expr Expr
```

To start, we will change the type of `eval`

so that it is effectful,
and supports an exception effect for throwing errors, and a state
containing a mapping from variable names (as `String`

) to their
values:

```
Env : Type
Env = List (String, Integer)
eval : Expr -> Eff Integer [EXCEPTION String, STATE Env]
eval (Val x) = return x
eval (Add l r) = return $ !(eval l) + !(eval r)
```

Note that we are using `!`

-notation to avoid having to bind
subexpressions in a `do`

block. Next, we add a case for evaluating
`Var`

:

```
eval (Var x) = case lookup x !get of
Nothing => raise $ "No such variable " ++ x
Just val => return val
```

This retrieves the state (with `get`

, supported by the `STATE Env`

effect) and raises an exception if the variable is not in the
environment (with `raise`

, supported by the `EXCEPTION String`

effect).

To run the evaluator on a particular expression in a particular
environment of names and their values, we can write a function which
sets the state then invokes `eval`

:

```
runEval : List (String, Integer) -> Expr -> Maybe Integer
runEval args expr = run (eval' expr)
where eval' : Expr -> Eff Integer [EXCEPTION String, STATE Env]
eval' e = do put args
eval e
```

We have picked `Maybe`

as a computation context here; it needs to be
a context which is available for every effect supported by
`eval`

. In particular, because we have exceptions, it needs to be a
context which supports exceptions. Alternatively, `Either String`

or
`IO`

would be fine, for example.

What if we want to extend the evaluator further, with random number
generation? To achieve this, we add a new constructor to `Expr`

,
which gives a random number up to a maximum value:

```
data Expr = Val Integer
| Var String
| Add Expr Expr
| Random Integer
```

Then, we need to deal with the new case, making sure that we extend
the list of events to include `RND`

. It doen’t matter where `RND`

appears in the list, as long as it is present:

```
eval : Expr -> Eff Integer [EXCEPTION String, RND, STATE Env]
eval (Random upper) = rndInt 0 upper
```

For test purposes, we might also want to print the random number which has been generated:

```
eval (Random upper) = do val <- rndInt 0 upper
putStrLn (show val)
return val
```

If we try this without extending the effects list, we would see an error something like the following:

```
Expr.idr:28:6:When elaborating right hand side of eval:
Can't solve goal
SubList [STDIO]
[(EXCEPTION String), RND, (STATE (List (String, Integer)))]
```

In other words, the `STDIO`

effect is not available. We can correct
this simply by updating the type of `eval`

to include `STDIO`

.

```
eval : Expr -> Eff Integer [STDIO, EXCEPTION String, RND, STATE Env]
```

Note that using `STDIO`

will restrict the number of contexts in
which `eval`

can be `run`

to those which support `STDIO`

, such
as `IO`

. Once effect lists get longer, it can be a good idea instead
to encapsulate sets of effects in a type synonym. This is achieved as
follows, simply by defining a function which computes a type, since
types are first class in Idris:

```
EvalEff : Type -> Type
EvalEff t = Eff t [STDIO, EXCEPTION String, RND, STATE Env]
eval : Expr -> EvalEff Integer
```