I have a file named "test.txt" with this text:

Good
Morning
Sir

and a file named "test.hs" with the following code:

module Main where

import System.IO

main :: IO ()
main = interact f

f :: String -> String
f s = head $ lines s

The following command...

cat test.txt | runhaskell test.hs

outputs

Good

However, I would like to explicitly pass parameters to runhaskell without relying on a file, like:

echo "Good\nMorning\nSir" | runhaskell test.hs

and also execute runhaskell with a literal string of Haskell code, like:

echo "Good\nMorning\nSir" | runhaskell "module Main where\nimport System.IO\nmain :: IO ()\nmain = interact f\nf :: String -> String\nf s = head $ lines s"

Is this technically possible ?

CodePudding user response：

The problem is that echo will output a backslash (\) and an n instead of a new line.

You can use the -e flag [unix.com], this flag will:

-e enable interpretation of backslash escapes

So we can pass a string with new lines to runhaskell's input channel with:

echo -e -- "Good\nMorning\nSir" | runhaskell test.hs

Note that cat test.txt | runhaskell test.hs is useless use of cat, you can replace this with:

runhaskell test.hs < test.txt

which is more efficient since we do not use a cat process or the pipe to pass data to.