Difference between revisions of "CTRex"

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(Introduction)
(Introduction)
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For example, we can declare shorthands for pointing at the type level symbol "x", "y" and "z" as follows.
 
For example, we can declare shorthands for pointing at the type level symbol "x", "y" and "z" as follows.
   
<haskell> x = Label :: Label "x"
+
<haskell>
  +
x = Label :: Label "x"
 
y = Label :: Label "y"
 
y = Label :: Label "y"
z = Label :: Label "z" </haskell>
+
z = Label :: Label "z"
  +
</haskell>
  +
  +
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.
  +
  +
Once we have declared some (pointers to) labels. We can write
  +
<code> { x = 0 , y = "bla", z = False } </code>
  +
as follows:
  +
<haskell> x := 0 .| y .= "bla" .| z := False .| empty </haskell>
  +
(again it would be nicer to just write <code> { x = 0 , y = "bla", z = False } </code>, but currently not possible).

Revision as of 16:19, 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.

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).