hadrian: Add --haddock-base-url option for specifying base-url when generating docs

The motiviation for this flag is to be able to produce documentation
which is suitable for uploading for hackage, ie, the cross-package links
work correctly.

There are basically three values you want to set this to:

* off - default, base_url = ../%pkg% which works for local browsing
* on - no argument , base_url = https:://hackage.haskell.org/package/%pkg%/docs - for hackage docs upload
* on - argument, for example, base_url = http://localhost:8080/package/%pkg%/docs for testing the documentation.

The `%pkg%` string is a template variable which is replaced with the
package identifier for the relevant package.

This is one step towards fixing #21749

1 files changed, 4 insertions, 0 deletions

diff --git a/hadrian/README.md b/hadrian/README.md
index 1a3335d535..d132ee0326 100644
--- a/hadrian/README.md
+++ b/hadrian/README.md
@@ -285,6 +285,10 @@ all of the documentation targets:
 You can pass several `--docs=...` flags, Hadrian will combine
 their effects.
 
+To build haddock documentation for upload to hackage you need to pass the `--haddock-base-url` flag,
+by default this will choose a url suitable for uploading to hackage but you might also want to pass something like
+`http://127.0.0.1:8080/package/%pkg%/docs` for testing upload locally on a local hackage server.
+
 #### Source distribution
 
 To build a GHC source distribution tarball, run `build source-dist`.