Is possible to output to stdout a simple Haskell one-liner like
main = print "Hello World"
directly from Bash? Something ala php -r 'echo "Hello World\n";'
I poked around in the ghc options but I didn't see anything that would help me.
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4 Answers
I think I figured it out from here
$ ghc -e "interact (unlines.map reverse.lines)"
hello
olleh
UPDATE:
just did some more tests, this works too:
echo "hi" | ghc -e "interact (unlines.map reverse.lines)"
// prints "ih"
Comments
Consider runhaskell command which can take piped stdin, for instance like this,
echo 'main = print "Hello World"' | runhaskell
Update
In general you can script Haskell source as follows,
#!/usr/bin/env runhaskell
main = putStrLn "Hello World"
This actually compiles and executes the program, whereas ghc -e will evaluate an expression.
3 Comments
Joe Crawfordy
Just tried this out and it seems to run super hella slow compared to
ghc -e, I wonder why that is? main = print "Hello" | runhaskell took 1.371s when I benchmarked it with time, compared to ghc -e '"Hello"' took .207s
Shanthakumar
using ghc -e also loads .ghci config but runhaskell doesn't.
Joe Crawfordy
Gotcha, I think most of the time I'd be looking for
ghc -e, but it's nice to know that I can do multi-lines if I need to with runhaskell (eg echo 'helloWorld = print "Hello World"; main = helloWorld' | runhaskell)You found ghc -e yourself. Here are some useful aliases that go well with that:
function hmap { ghc -e "interact ($*)"; }
function hmapl { hmap "unlines.($*).lines" ; }
function hmapw { hmapl "map (unwords.($*).words)" ; }
(Discussed in this old blog post of mine.)
Comments
I wrote eddie (https://eddie.googlecode.com/) to provide more facilities than "-e". From the linked page:
Eddie adds features to make using it for shell scripting easier:
When given file arguments, eddie feeds them to your function. Eddie can easily add modules to the namespace you use. Eddie has options for processing things a line or file at a time. Eddie will use binary file IO methods when asked to.
Be warned that eddie was my first "real" haskell application, and is seriously in need of a rewrite.
