# DDC/PolymorphicUpdate

**This is outdated information**. The DDC project has moved to http://discus-lang.org

## Get/Set

Consider the following function:

```
makeGetSet :: forall a. a -> (() -> a, a -> ())
makeGetSet x
= do box = Just x
get () = case box of { Just z -> z; }
set z = box#x #= z
(get, set)
```

This function allocates a box which can store a value, and returns a tuple of functions to get and set that value.

As the function is polymorphic, we can create boxes of whatever type we would like:

```
main ()
= do getSet :: (() -> Int, Int -> ())
getSet = makeGetSet 5
out $ fst getSet () -- prints '5'
snd getSet 23 -- update the box
out $ fst getSet () -- prints '23'
```

The trouble comes when we create a box containing a value of polymorphic type. Without closure typing we could define:

```
...
getSet2 :: forall a. (() -> [a], [a] -> ())
getSet2 = makeGetSet []
```

When a list is empty, we can treat it as being of any type `(forall a. [a])`

, but suppose we update the box containing it at two different types...

```
snd getSet2 [23]
snd getSet2 ["trouble"]
out $ fst getSet2 ()
```

The type of `getSet2`

has `forall a.`

at the front, so there is nothing to stop us from calling the set function at both `[Int]`

and `[String]`

, but what should the type be when use the get function in the last line?

## Dangerous type variables

Ultimately, the problem illustrated above arose because there wasn't a mechanism to track the sharing of data between successive calls to `getSet2`

. When `makeGetSet`

was evaluated it created a shared mutable object (the `box`

) and then returned functions that had this object free in their closure.

In Disciple, `makeGetSet`

has the full type:

```
makeGetSet
:: forall a %r0 %r1
. a -> Tuple2 %r1 (() -(!e0 $c0)> a) (a -(!e1 $c1)> ())
:- !e0 = !Read %r0
, !e1 = !{!Read %r0; !Write %r0}
, $c0 = ${box : %r0; box : %r0 $> a}
, $c1 = ${box : %r0; box : %r0 $> a}
, Base.Mutable %r0
```

In this type, we see the new closure term `(box : %r0 $> a)`

. This term says that the closure contains an object named `box`

which is in a region `%r0`

, and the type of the object includes a variable `'a'`

. When `%r0`

is `Mutable`

we say that `a`

is *dangerous*, and dangerous variables are never generalised when they are free in the (outer most) closure of a function.