If you want your output dir as anything other than /output, then using this is fine:

spago build -- --output outputDir

However this doesn't work...

spago bundle-app --to index.js -- --output outputDir

...because although it compiles the .purs files into outputDir, It then runs the purs bundle command with "output" hardcoded:

Running command: `purs bundle "output/*/*.js" -m Main --main Main -o index.js`
purs bundle: No input files.

I guess the solution would be either to detect the --output/-o args passed to the purs compiler, or possibly to add an --output option to spago itself? An entry in spago.dhall would also be nice, if you're going that way.