# Difference between revisions of "CTRex"

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* Fast extend, lookup and restriction (all O(log n)) using HashMaps. |
* Fast extend, lookup and restriction (all O(log n)) using HashMaps. |
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+ | The haddock documentation is available [http://homepages.cwi.nl/~ploeg/openrecdocs/Records.html here]. |
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Labels (such as x,y and z) in CTRex are type level symbols (i.e. type level strings). We can point to a label by using the label type: |
Labels (such as x,y and z) in CTRex are type level symbols (i.e. type level strings). We can point to a label by using the label type: |
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Of course it would be much nicer to just write <hask>`x</hask> instead of <hask> Label :: Label "x"</hask> but this is currently not available. This may change in the future. |
Of course it would be much nicer to just write <hask>`x</hask> instead of <hask> Label :: Label "x"</hask> but this is currently not available. This may change in the future. |
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− | Once we have declared some (pointers to) labels. We can write |
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− | <code> { x = 0 , y = "bla", z = False } </code> |
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− | as follows: |
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− | <haskell> x := 0 .| y .= "bla" .| z := False .| empty </haskell> |
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− | (again it would be nicer to just write <code> { x = 0 , y = "bla", z = False } </code>, but currently not possible). |
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For example, the value level operation for extending a record (adding a field) has type <hask> extend :: KnownSymbol l => Label l -> a -> Rec r -> Rec (Extend l a r) </hask> wheras the type level operation for adding a field has type <hask> Extend (l :: Symbol) (a :: *) (r :: Row *) :: Row *</hask>. In this way each value level operation (that changes the type) has a corresponding type level operation with the same name (but starting with a capitol). |
For example, the value level operation for extending a record (adding a field) has type <hask> extend :: KnownSymbol l => Label l -> a -> Rec r -> Rec (Extend l a r) </hask> wheras the type level operation for adding a field has type <hask> Extend (l :: Symbol) (a :: *) (r :: Row *) :: Row *</hask>. In this way each value level operation (that changes the type) has a corresponding type level operation with the same name (but starting with a capitol). |

## Revision as of 16:43, 4 December 2013

# Introduction

This page will describe the design, usage and motivation for CTRex.

CTRex is a library for Haskell which implements extensible records using closed type families and type literals. It does **not** use overlapping instances.

Features:

- Row-polymorphism

- Support for scoped labels (i.e. duplicate labels)
**and**non-scoped labels (i.e. the lacks predicate on rows).

- The value level interface and the type level interface correspond to each other.

- The order of labels (except for duplicate labels) does not matter. I.e. {x = 0, y = 0} and {y = 0, x = 0} have the
**same type**.

- Syntactic sugar on the value level as well as type level.

- If all values in a record satisfy a constraint such as
`Show`

, then we are able to do operations on all fields in a record, if that operation only requires that the constraint is satisfied. In this way we can create instances such as`Forall r Show => Show (Rec r)`

. This is available to the application programmer as well.

- Fast extend, lookup and restriction (all O(log n)) using HashMaps.

The haddock documentation is available here.

### Labels

Labels (such as x,y and z) in CTRex are type level symbols (i.e. type level strings). We can point to a label by using the label type:

```
data Label (s :: Symbol) = Label
```

For example, we can declare shorthands for pointing at the type level symbol "x", "y" and "z" as follows.

```
x = Label :: Label "x"
y = Label :: Label "y"
z = Label :: Label "z"
```

Of course it would be much nicer to just write ``x`

instead of `Label :: Label "x"`

but this is currently not available. This may change in the future.

### Extension

For example, the value level operation for extending a record (adding a field) has type `extend :: KnownSymbol l => Label l -> a -> Rec r -> Rec (Extend l a r)`

wheras the type level operation for adding a field has type `Extend (l :: Symbol) (a :: *) (r :: Row *) :: Row *`

. In this way each value level operation (that changes the type) has a corresponding type level operation with the same name (but starting with a capitol).

## Syntactic sugar

```
p :<-| z .| y :<- 'b' .| z :!= False .| x := 2 .| y := 'a' .| empty
```

instead of

```
rename z p $ update y 'b' $ extendUnique z False $ extend x 2 $ extend y 'a' empty
```