include extensions in haskell-stack conf file

Question

I want to add ViewPatterns extensions to my build and currently I do it by adding the following lines to package.yaml

ghc-options:
- -Wall
- -XViewPatterns

I get a warning:

Warning: Instead of 'ghc-options: -XViewPatterns' use 'extensions:
ViewPatterns'

But, when I add an extensions field to package.yaml, I get the following:

WARNING: Ignoring unknown field "extensions" in package description

Also, I could not find any definition of extensions in the official stack documentation.

Answer 1

Do not confuse stack with hpack.

package.yaml is actually read by hpack. But stack build command implicitly calls hpack to automatically convert into a cabal file, which may make you confused.

Anyway, to specify extensions in package.yaml, use default-extensions:

default-extensions: ViewPatterns

Answer 2

extensions: is currently unsupported by stack, see follow-ups in this github issue.