Simple Scripts

Simplest script in haskell looks like this: [simple.hs](./simple.hs):

#!/usr/bin/env stack
-- stack --resolver=lts-12.7 script

main :: IO ()
main = do
  putStrLn "hello, scripting!"

Just mark this file as executable (via chmod +x ./simple_stdin_stdout.hs) and launch as usual:

$ ./simple_stdin_stdout.hs
hello, scripting!

You can compile your script to get an executable like this:

$ stack ghc --resolver=lts-12.7 ./simple_stdin_stdout.hs
Linking simple_stdin_stdout ...
$ ./simple_stdin_stdout
hello, scripting!

With a stack script shown above, you can use whatever packages are present in a snapshot without listing them out. For example: simple_with_dep.hs:

#!/usr/bin/env stack
-- stack --resolver=lts-12.7 script

import Database.Redis

main :: IO ()
main = do
  putStrLn "hello, scripting!"

If you want to use a non-snapshot dependency, you can use a stack runghc (or stack ghc) command with dependency packages being listed in a --package argument: simple_stdin_stdout_nonsnapshot_dep.hs:

#!/usr/bin/env stack
-- stack --resolver=lts-12.7 --package corenlp-parser script

import NLP.CoreNLP

main :: IO ()
main = do
  putStrLn "hello, scripting!"

Notes/Caveats:

• TODO compiling with Wall/Werror and other flags
• buffering (per-line? per-byte? need to set buffering mode)
• best practice: upon reading/writing to stdin/stdout, usage of String is bad, better use Text, best – bytestrings (TODO: citation on encoding problems needed)