Typed Template Haskell

Main.hs

{-# LANGUAGE TemplateHaskell #-}

import TH

main :: IO ()
main = do
  let numbers = $$(primesUpTo' 10000)
  putStrLn "Which prime number do you want to know?"
  input <- readLn
  if input < length numbers
    then print (numbers !! (input - 1))
    else putStrLn "Number too big!"

Primes.hs

module Primes where

isPrime :: Integer -> Bool
isPrime n
  | n <= 1    = False
  | n == 2    = True
  | even n    = False
  | otherwise = go 3
  where
    go i
      | i >= n         = True
      | n `mod` i == 0 = False
      | otherwise      = go (i + 2)

primesUpTo :: Integer -> [Integer]
primesUpTo n = go 2
  where
    go i
      | i > n     = []
      | isPrime i = i : go (i + 1)
      | otherwise = go (i + 1)

TH.hs

{-# LANGUAGE TemplateHaskell #-}

module TH where

import Language.Haskell.TH

import Primes (isPrime)

primesUpTo' :: Integer -> Q (TExp [Integer])
primesUpTo' n = go 2
  where
    go i
      | i > n     = [||[]||]
      | isPrime i = [||i : $$(go (i + 1))||]
      | otherwise = go (i + 1)