uom-plugin difference

Examples.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

{-# OPTIONS_GHC -fplugin Data.UnitsOfMeasure.Plugin #-}
module Examples where

import Data.UnitsOfMeasure
import Data.UnitsOfMeasure.Convert
import Data.UnitsOfMeasure.Show


[u| kg |]

myMass = [u| 65.2 kg |]

execution-result.txt

(7.10.1)nushio@nuwheezy:uom-plugin$ runhaskell Main.hs
[u| 65.2 kg |]
(7.10.1)nushio@nuwheezy:uom-plugin$ ghci Main.hs
GHCi, version 7.10.1: http://www.haskell.org/ghc/  :? for help
[1 of 2] Compiling Examples         ( Examples.hs, interpreted )
[2 of 2] Compiling Main             ( Main.hs, interpreted )
Ok, modules loaded: Examples, Main.
> main
[u| 65.2 kg |]
> print myMass 

<interactive>:3:1:
    No instance for (Data.UnitsOfMeasure.Singleton.KnownUnit
                       (Unpack (Base "kg")))
      arising from a use of ‘print’
    In the expression: print myMass
    In an equation for ‘it’: it = print myMass
>  :seti -fplugin Data.UnitsOfMeasure.Plugin 
> print myMass
[u| 65.2 kg |]
>

Main.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}

{-# OPTIONS_GHC -fplugin Data.UnitsOfMeasure.Plugin #-}

import Data.UnitsOfMeasure
import Data.UnitsOfMeasure.Convert
import Data.UnitsOfMeasure.Show

import Examples

main :: IO ()
main = print myMass

I wonder why the results of evaluating main and print myMass differ, given that main = print myMass .

Solution: to activate the type checker plugin in the GHCi. :seti -fplugin Data.UnitsOfMeasure.Plugin . As we can see, when we load a .hs file to GHCi, the plagmas such as type checker plugins and LANGUAGE pragrmas are not activated.