Using `runhaskell` with command-line arguments

Question

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 ?

willeM_ Van Onsem · Accepted Answer · 2022-01-08 15:59:57Z

2

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.

edited Jan 8, 2022 at 15:59

answered Jan 8, 2022 at 15:54

willeM_ Van Onsem

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3 Comments

F. Zer Over a year ago

Thank you so much. Why do you have two dashes after -e ?

willeM_ Van Onsem Over a year ago

@F.Zer: that is to prevent that the content of the string, is interpreted as a flag. Here that is not the case, but imagine that the string for example is -n, then echo parses this as a flag. By using two dashes, it is seen as a parameter: unix.stackexchange.com/questions/11376/…

F. Zer Over a year ago

Thank you ! Do you know whether my second concern is technically possible ?

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