SeasonalFruitsを「SeasonとFruitのタプル」のリストと定義、即ち、季節と果物の直積の部分集合を表す。SeasonalFruits -> [Season] という型から、query関数はSeasonalFruitsから季節の部分集合への写像であると読める。

gistfile1.hs

module Main where
import Control.Monad

type Season = String
type Fruit = String
type SeasonalFruits = [(Season, Fruit)]

dt :: SeasonalFruits
dt =
  [ ("春", "いちご")
  , ("夏", "もも")
  , ("春", "りんご")
  , ("冬", "みかん")
  ]

query :: SeasonalFruits -> [Season]
query list = do
  (season, fruit) <- list
  guard $ fruit == "もも"
  return season

main :: IO ()
main = mapM_ putStrLn (query dt)