The collection of service parameters carried in a single SVCB or HTTPS record, keyed by SVCParamKey (one value per key code, per RFC 9460). Construction is via the IsList instance — fromList [SVCParamValue p1, ...]; enumeration via toList returns the parameters in ascending key order, which is also the canonical on-the-wire order.

Each key code is associated with a specific KnownSVCParamValue type; the values are held inside the existential SVCParamValue wrapper so a single set can mix heterogeneous parameter types. spvLookup takes the parameter type as a type application and returns the typed value when the matching key is present:

ghci> import qualified GHC.IsList as IL (fromList)
ghci> spvset = IL.fromList @SPVSet [SVCParamValue (SPV_PORT 80), SVCParamValue SPV_NDALPN]
ghci> spvset
fromList @SPVSet [SVCParamValue SPV_NDALPN,SVCParamValue 80]
ghci> spvLookup @SPV_ndalpn spvset
Just SPV_NDALPN

The type application can be elided when a pattern match constrains the result type:

ghci> case spvLookup spvset of { Just (SPV_PORT p) -> p; _ -> 0 }
80