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.
What are extensible records?
A record is just a collection of label-value pairs. For example:
{ x = 0 , y = "bla", z = False }
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).