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
| Construct a record | > (#foo := "hi", #bar := 123) (#foo := "hi",#bar := 123) |
| Get fields of a record | > get #bar (#foo := "hi", #bar := 123) 123 |
| Set fields of a record | > set #bar 66 (#foo := "hi", #bar := 123) (#foo := "hi",#bar := 66) |
| Modify fields of a record | > modify #mu (*0.1) (#bar := "hi", #mu := 123) (#bar := "hi",#mu := 12.3) |
| Add fields to a record | > cons (#mu := [1,2,3]) (#foo := "hi", #bar := 123) (#mu := [1,2,3],#foo := "hi",#bar := 123) |
| Abstraction | > let double field record = set field (get field record * 2) record > double #mu (#bar := "hi", #mu := 123) (#bar := "hi",#mu := 246) |
Reading CSV files with Cassava
Import the instances for FromNamedRecord:
import Labels.Cassava
Then just specify the type you want to load:
> let Right (_,rows :: Vector ("salary" := Int, "name" := Text)) = decodeByName "name,salary\r\nJohn,27\r\n" > rows [(#salary := 27,#name := "John")]
Non-existent fields or invalid types result in a parse error:
> decodeByName "name,salary\r\nJohn,27\r\n" :: Either String (Header, Vector ("name" := Text, "age" := Int)) Left "parse error (Failed reading: conversion error: Missing field age) at \"\\r\\n\"" > decodeByName "name,salary\r\nJohn,27\r\n" :: Either String (Header, Vector ("name" := Text, "salary" := Char)) Left "parse error (Failed reading: conversion error: expected Char, got \"27\") at \"\\r\\n\""
Example with Yahoo!'s market data for AAPL:
> Right (headers,rows :: Vector ("date" := String, "high" := Double, "low" := Double)) <- fmap decodeByName (LB.readFile "AAPL.csv") > headers ["date","open","high","low","close","volume","adj close"]
We can print the rows as-is:
> mapM_ print (V.take 2 rows) (#date := "2016-08-10",#high := 108.900002,#low := 107.760002) (#date := "2016-08-09",#high := 108.940002,#low := 108.010002)
Accessing fields is natural as anything:
> V.sum (V.map (get #low) rows) 2331.789993
We can just make up new fields on the fly:
> let diffed = V.map (\row -> cons (#diff := (get #high row - get #low row)) row) rows > mapM_ print (V.take 2 diffed) (#diff := 1.1400000000000006,#date := "2016-08-10",#high := 108.900002,#low := 107.760002) (#diff := 0.9300000000000068,#date := "2016-08-09",#high := 108.940002,#low := 108.010002)
Sometimes a CSV file will have non-valid Haskell identifiers or spaces, e.g. adj close here:
> Right (headers,rows :: Vector ("date" := String, "adj close" := Double)) <- fmap decodeByName (LB.readFile "AAPL.csv") > mapM_ print (V.take 2 rows) (#date := "2016-08-10",#adj close := 108.0) (#date := "2016-08-09",#adj close := 108.809998)
Just use the $("adj close") syntax:
> mapM_ print (V.take 2 (V.map (get $("adj close")) rows)) 108.0 108.809998
It still checks the name and type:
> mapM_ print (V.take 2 (V.map (get $("adj closer")) rows)) <interactive>:133:31: error: • No instance for (Has "adj closer" a0 ("date" := String, "adj close" := Double)) arising from a use of ‘get’
