convention around pattern synonyms

Richard Eisenberg lists at
Thu Dec 30 16:46:29 UTC 2021

I agree that this kind of backward-compatibility pattern synonym is good and shouldn't be prefixed with PS_.

But do you have a concrete example of this leakage of an internal GHC type via TH? While I can imagine this happening, I don't know of any examples in practice. Note that even enumeration types (like Role) have separate TH counterparts.


> On Dec 29, 2021, at 6:12 PM, Viktor Dukhovni <ietf-dane at> wrote:
> Some "GHC-internal" types leak to users via TH, and their constructors
> occasionally pick up new fields, causing breakage downstream.  The extra
> field often has a sensible default (Nothing, [], ...) and it should be
> best practice to rename the constructor when adding the new field, while
> replacing the original constructor with a pattern synonym with the "old"
> signature.
> 	data Foo = ...
> 		 | NewImprovedMkFoo X Y Z -- was MkFoo Y Z
> 	pattern MkFoo :: Foo
> 	pattern MkFoo Y Z = NewImprovedMkFoo Nothing Y Z
> When pattern synonyms are used to maintain a backwards-compatible API,
> there should of course be no special signalling to differentiate them
> from "real" constructors.
> The boundary between "GHC-internal" and external may not always be
> obvious, some care is required to reduce leaking breakage via TH.
> -- 
> 	Viktor.
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