Online REPL Walkthrough

In the online REPL, you can use the editor pane on the left hand side to insert and edit Frege code.

You can then run it by clicking the 'Evaluate' button at the bottom or by pressing 'Ctrl + Enter'.

This wiki page should provide you with some ideas that you may want to try in the Repl.

Please note that at the moment any printing to stdout does not lead to visible output. Only the value of the last evaluated expression is visible.

Converting to JSON

Copy the following code in the editor and evaluate it.

This will show the :load editor command in the repl and some evaluation result. You can now run the run function by typing run in the repl.

module examples.JSONExample where

import Data.JSON

data Coordinate = Coordinate { lon :: Float, lat :: Float }
derive Show Coordinate

data Weather = Weather { loc :: Coordinate
                       , temp :: Int
                       , desc :: String
                       , alert :: Maybe String
                       }
derive Show Weather

instance ToJSON Coordinate where
  toJSON coord = Struct [ ("lon", toJSON coord.lon), ("lat", toJSON coord.lat) ]


instance ToJSON Weather where
  toJSON Weather{loc, temp, desc, alert}
    = Struct [ ("loc", toJSON loc)
             , ("temp", toJSON temp)
             , ("desc", toJSON desc)
             , ("alert", toJSON $ fromMaybe "null" alert)
             ]

run = toJSON $ Weather location 98 "overcast clouds" (Just "Excessive heat")
  where
    location = Coordinate { lon = -122.09, lat = 37.39 }