# 4. Constraining constructors with `:<:`

¶

The `:<:`

type operator is used to ensure that a constructor is present in an
EADT. For example if we consider the following type signature (that will be
developed in the example below):

```
distr :: (AddF :<: f, MulF :<: f) => EADT f -> Maybe (EADT f)
```

The constructors of `EADT f`

are not specified but the constraints ```
(AddF :<:
f, MulF :<: f)
```

ensure that at least `AddF`

and `MulF`

constructors are present.

Note that to shorten a list of constraints such as `(AddF :<: f, MulF :<: f)`

you can use the `:<<:`

operator: `'[AddF,MulF] :<<: f`

.

## 4.1. Transformation example¶

Suppose we have the following EADT for arithmetic expressions:

```
{-# LANGUAGE DeriveFunctor #-}
data ValF e = ValF Int deriving (Functor)
data AddF e = AddF e e deriving (Functor)
data MulF e = MulF e e deriving (Functor)
eadtPattern 'ValF "Val"
eadtPattern 'AddF "Add"
eadtPattern 'MulF "Mul"
type Expr = EADT '[ValF, AddF, MulF]
```

We can define some value:

```
e1 :: Expr
e1 = Add (Val 10)
(Mul (Add (Val 5)
(Val 10))
(Val 7))
```

We can define instances of the `MyShow`

class (defined here):

```
instance MyShow (ValF e) where
myShow (ValF e) = show e
instance MyShow e => MyShow (AddF e) where
myShow (AddF x y) = "(" ++ myShow x ++ " + " ++ myShow y ++ ")"
instance MyShow e => MyShow (MulF e) where
myShow (MulF x y) = "(" ++ myShow x ++ " * " ++ myShow y ++ ")"
> putStrLn (myShow e1)
(10 + ((5 + 10) * 7))
```

Now we can define a transformation that distributes multiplication over addition as follows:

```
-- distribute multiplication over addition if it matches
distr :: (AddF :<: f, MulF :<: f) => EADT f -> Maybe (EADT f)
distr (Mul a (Add c d)) = Just (Add (Mul a c) (Mul a d))
distr (Mul (Add c d) a) = Just (Add (Mul c a) (Mul d a))
distr _ = Nothing
```

Note that this function works on any EADT as long as it has `AddF`

and
`MulF`

constructors. We indicate such constraints with the `:<:`

type
operator.

Then we need a helper function that performs the traversal of the EADT:

```
import Control.Arrow
-- bottom up traversal that performs an additional bottom up traversal in
-- the transformed sub-tree when a transformation occurs.
bottomUpFixed :: Functor (VariantF cs) => (EADT cs -> Maybe (EADT cs)) -> EADT cs -> EADT cs
bottomUpFixed f = unfix >>> fmap (bottomUpFixed f) >>> Fix >>> f'
where
f' u = case f u of
Nothing -> u
Just v -> bottomUpFixed f v
```

Note

`bottomUpFixed`

is a generic recursion scheme over an EADT. You can read more on this approach in the dedicated chapter.

Finally we can test the transformation on an example:

```
> putStrLn (myShow e1)
(10 + ((5 + 10) * 7))
> putStrLn (myShow (bottomUpFixed distr e1))
(10 + ((5 * 7) + (10 * 7)))
```

### 4.1.1. Extensibility¶

Suppose we add a `Pow`

(power) constructor:

```
data PowF e = PowF e e deriving (Functor)
eadtPattern 'PowF "Pow"
instance MyShow e => MyShow (PowF e) where
myShow (PowF x y) = "(" ++ myShow x ++ " ^ " ++ myShow y ++ ")"
```

We can now write expressions that use the `Pow`

constructor:

```
type Expr2 = EADT '[ValF, AddF, MulF, PowF]
e2 :: Expr2
e2 = Pow (Val 10)
(Mul (Add (Pow (Val 5) (Val 8))
(Val 10))
(Val 7))
```

We can check that our distribution function still works on this new type of expression without being modified at all:

```
> putStrLn (myShow (bottomUpFixed distr e2))
(10 ^ (((5 ^ 8) * 7) + (10 * 7)))
```