Question

I have a file with some data in it. This data never changes and I want to make it available outside of the IO monad. How can I do that?

Example (note that this is just an example, my data is not computable):

primes.txt:

2 3 5 7 13

code.hs:

primes :: [Int]
primes = map read . words . unsafePerformIO . readFile $ "primes.txt"

Is this a “legal” use of unsafePerformIO? Are there alternatives?

Answer 1

You could use TemplateHaskell to read in the file at compile time. The data of the file would then be stored as an actual string in the program.

In one module (Text/Literal/TH.hs in this example), define this:

module Text.Literal.TH where

import Language.Haskell.TH
import Language.Haskell.TH.Quote

literally :: String -> Q Exp
literally = return . LitE . StringL

lit :: QuasiQuoter
lit = QuasiQuoter { quoteExp = literally }

litFile :: QuasiQuoter
litFile = quoteFile lit

In your module, you can then do:

{-# LANGUAGE QuasiQuotes #-}
module MyModule where

import Text.Literal.TH (litFile)

primes :: [Int]
primes = map read . words $ [litFile|primes.txt|]

When you compile your program, GHC will open the primes.txt file and insert its contents where the [litFile|primes.txt|] part is.