CTRex
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. 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 typeExtend (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). - 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 to write
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
- 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 satifies. In this way we can create instanstances such asForall r Show => Show (Rec r)
. This is available to the application programmer as well.
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.
Once we have declared some (pointers to) labels. We can write
{ x = 0 , y = "bla", z = False }
as follows:
x := 0 .| y .= "bla" .| z := False .| empty
(again it would be nicer to just write { x = 0 , y = "bla", z = False }
, but currently not possible).