1. Calling R in Haskell

    2016-09-03
    Source

    This is a demo of the inline-R Haskell library, allowing to call R from Haskell.

    In order to call R in GHCi, you have to run this preamble before:

    :set -XDataKinds
:set -XGADTs
:set -XPartialTypeSignatures
:set -XOverloadedLists
:set -XQuasiQuotes
:set -XScopedTypeVariables
:set -XTemplateHaskell
:set -XViewPatterns
import Data.Int (Int32)
import H.Prelude.Interactive
import qualified Language.R.Instance as R
R.initialize R.defaultConfig

    Therefore, since I use knitr+ghcscriptrender to write this article, I save the following code in a file Rpreamble.hs:

    import Data.Int (Int32)
import H.Prelude.Interactive
import qualified Language.R.Instance as R
 
main :: IO()
main = do
  R.initialize R.defaultConfig
  return()

    And now I can call R like this:

    > :load Rpreamble.hs
> :set -XTemplateHaskell -XQuasiQuotes -XScopedTypeVariables -XDataKinds -XGADTs -XPartialTypeSignatures -XOverloadedLists -XViewPatterns
> -- perform 1+1 in R:
> x <- [r| 1 + 1 |]
> fromSomeSEXP x :: Double
2.0
> -- integers from 1 to 3:
> x <- [r| 1:3 |]
> fromSomeSEXP x :: [Int32]
[1,2,3]

    One can run several commands in [r| ...|]:

    > :load Rpreamble.hs
> :set -XTemplateHaskell -XQuasiQuotes -XScopedTypeVariables -XDataKinds -XGADTs -XPartialTypeSignatures -XOverloadedLists -XViewPatterns
> -- separate with a semi-colon:
> x <- [r| a <- 2; 3+a |]  
> fromSomeSEXP x :: Double
5.0
> -- or multiline:
> :{
> x <- [r| a <- 2
> 3+a |]
> :}
> fromSomeSEXP x :: Double
5.0

    One can load and use a package:

    > :load Rpreamble.hs
> :set -XTemplateHaskell -XQuasiQuotes -XScopedTypeVariables -XDataKinds -XGADTs -XPartialTypeSignatures -XOverloadedLists -XViewPatterns
> -- load and use the jsonlite package
> :{
> x <- [r| library(jsonlite)
> dat <- data.frame(name=c('Joe', 'Bill'), age=c(23,40))
> toJSON(dat) |]
> :}
> fromSomeSEXP x :: String
"[{\"name\":\"Joe\",\"age\":23},{\"name\":\"Bill\",\"age\":40}]"
> -- But I prefer this way sometimes:
> :{
> x <- [r| dat <- data.frame(name=c('Joe', 'Bill'), age=c(23,40))
> jsonlite::toJSON(dat) |]
> :}
> fromSomeSEXP x :: String
"[{\"name\":\"Joe\",\"age\":23},{\"name\":\"Bill\",\"age\":40}]"

    The most interesting feature is the ability to pass a Haskell variable in R:

    > :load Rpreamble.hs
> :set -XTemplateHaskell -XQuasiQuotes -XScopedTypeVariables -XDataKinds -XGADTs -XPartialTypeSignatures -XOverloadedLists -XViewPatterns
> -- 
> let a = 2::Double
> x <- [r| 3 + a_hs|]
> fromSomeSEXP x :: Double
5.0

    The p function displays the output of R???s print function:

    > :load Rpreamble.hs
> :set -XTemplateHaskell -XQuasiQuotes -XScopedTypeVariables -XDataKinds -XGADTs -XPartialTypeSignatures -XOverloadedLists -XViewPatterns
> -- print
> p [r| 1+1 |]
[1] 2

    The jsonlite package allows to get arrays and lists from Haskell:

    > :load Rpreamble.hs
> :set -XTemplateHaskell -XQuasiQuotes -XScopedTypeVariables -XDataKinds -XGADTs -XPartialTypeSignatures -XOverloadedLists -XViewPatterns
> -- an array:
> let a = [[1,2],[3,4]]::[[Int]]
> let b = show a
> p [r| jsonlite::fromJSON(b_hs) |]
     [,1] [,2]
[1,]    1    2
[2,]    3    4
> -- a list:
> let a = [[1,2],[3,4,5]]::[[Int]]
> let b = show a
> p [r| jsonlite::fromJSON(b_hs) |]
[[1]]
[1] 1 2

[[2]]
[1] 3 4 5

    And dataframes, with the help of the aeson library.

    > :load Rpreamble.hs
> :set -XTemplateHaskell -XQuasiQuotes -XScopedTypeVariables -XDataKinds -XGADTs -XPartialTypeSignatures -XOverloadedLists -XViewPatterns
> -- get the JSON representation of a dataframe:
> import Data.Aeson
> import GHC.Generics
> :set -XOverloadedStrings -XDeriveGeneric
> data Person = Person { name :: String, age :: Int } deriving (Show, Generic)
> instance ToJSON Person
> let persons = [Person "Joe" 21, Person "Bill" 40]::[Person]
> encode persons
"[{\"age\":21,\"name\":\"Joe\"},{\"age\":40,\"name\":\"Bill\"}]"
> -- pass it to R:
> import qualified Data.Text.Lazy.Encoding as T
> import qualified Data.Text.Lazy as T
> let b = T.unpack $ T.decodeUtf8 $ encode persons
> p [r| jsonlite::fromJSON(b_hs) |]
  age name
1  21  Joe
2  40 Bill