labels

Declare and access tuple fields with labels

This package is experimental, exploring the design space opened up by the implemented and to-be-implemented work on extensible records in GHC.

Note: You need GHC 8.0.1 for the #foo syntax, otherwise you have to use $("foo") which works on GHC 7.10.

Basic examples

The haddock docs are here.

Enable these extensions:

• In GHCi: :set -XOverloadedLabels -XTypeOperators -XDataKinds -XFlexibleContexts

• In a module: {-# LANGUAGE OverloadedLabels, TypeOperators, DataKinds, FlexibleContexts #-}

Let's use GHCi:

> import Labels
> :set -XOverloadedLabels -XTypeOperators -XDataKinds -XFlexibleContexts

> (#foo := "hi", #bar := 123)
(#foo := "hi",#bar := 123)

> get #bar (#foo := "hi", #bar := 123)
123

> set #bar 66 (#foo := "hi", #bar := 123)
(#foo := "hi",#bar := 66)

> modify #mu (*0.1) (#bar := "hi", #mu := 123)
(#bar := "hi",#mu := 12.3)

> cons (#mu := [1,2,3]) (#foo := "hi", #bar := 123)
(#mu := [1,2,3],#foo := "hi",#bar := 123)

> let double field record = set field (get field record * 2) record
> double #mu (#bar := "hi", #mu := 123)
(#bar := "hi",#mu := 246)