How to run a Haskell file in interpreted mode

76
votes

I've been told you can interpret Haskell files (which I assume means they will work like Ruby/Python/Perl). I can't find the command line option on GHC to do this, though. It always wants to compile my file. Took a look at GHCi as well, but it always dumps me into a repl.

I'm basically wanting to just do ghc -i MyFile.hs (where -i is a made up flag that I'm pretending correllates to interpreted mode) and have it execute so that I can get quick feedback while I'm trying out ideas and learning.

4
haskellghcghciinterpreted-language
There's a lot to like about ghci: it makes it very easy to test small parts of your program. I personally am just in the habit of using ghci to test things. You can still run the top-level main action by typing main (or using :main if you need to pass it command-line arguments). – Daniel Wagner

4 Answers

128
votes
$ runhaskell MyFile.hs

Alternatively, runghc (they're the same thing). ghci MyFile.hs will also start an interactive REPL session with MyFile.hs loaded, but if you want to run a main program then runhaskell is the way to go.

It's probably a good idea to get into the habit of testing parts of your program as isolated units in GHCi rather than running the whole thing each time, but obviously for shorter scripts it's simplest and easiest just to run the entire thing.

40
votes

You can have a script like this:

#!/usr/bin/env runhaskell
main = putStrLn "hello world"

After making the file executable (ie chmod +x haskell_script), you can run it like any other shell script.

11
votes

Open the GHC interpreter by running ghci in a terminal, and then load a file typing :load example.hs. More details in this link.

1
votes

To run the code written in a file, say myfile.txt, containing simple lines of code which work in the GHC interpreter, like:

let a = 0 in a:[1,2]
let x = [1,2] in x ++ [3,4]

you can do:

ghc -e ':script myfile.txt'

Edit

On Windows, double quotes are required:

ghc -e ":script myfile.txt"

Instead, one can also open GHCi and do :script myfile.txt.