PureScript programs export a main function which is invoked when you run the program. But it's unclear how to pass arguments to it, or if that is possible.
Consider the following simple program, which capitalizes a string.
module Main where
import Effect (Effect)
import Effect.Console (log)
import Prelude (Unit, discard)
import Data.String (toUpper)
message :: String
message = "Satisfaction Rating: 8.23"
main ∷ Effect Unit
main = do
log ( toUpper ( message ) )
Which outputs
SATISFACTION RATING: 8.23
This program is executed when spago run is invoked.
My question is as follows:
How can the argument message in the code example, be passed in via the command line, creating a custom message capitalizer?
The updated run command would look something like:
spago run --message="new_message"
And the updated output would look like
NEW_MESSAGE
In short, how do you pass values to a PureScript program from outside?
When you execute a PureScript program with spago run, it runs under Node. Therefore, to get command-line arguments, you need to as Node.
And look: there is a PureScript binding for the relevant Node facilities - the argv function from purescript-node-process. It's an effect that returns an array of arguments:
import Node.Process (argv)
main = do
args <- argv
log $ show args
A quick examination of spago run --help reveals that arguments can be passed to the program via the -b parameter:
spago run -b somearg
For multiple arguments, quote them:
spago run -b "somearg anotherarg"
Note that this works on Linux (and presumably macOS), but fails on Windows, where Spago for some reason ignores the quotes and tries to interpret anotherarg as its own argument. This is clearly a bug. This issue seems relevant.