Norman Ramsey writes:

Type StgOp and its constructors are exported from module GHC.Stg.Syntax. But the prettyprinting function pprStgOp is not exported. I'm thinking of exporting the function and also of defining an instance of Outputable. Is there any reason not to?

If this sounds OK to people, I'll make a PR.

An Outputable instance seems quite reasonable to me. Exporting pprStgOp seems less necessary but I wouldn't object. Cheers, - Ben