The example for GHC Generics is kinda broken
From http://www.reddit.com/r/haskell/comments/2douzn/problem_with_popular_ghcgenerics_example/:
The popular GHC.Generics serialization example at http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-programming.html\#idp25226064 illustrates how to serialize a sum datatype (a :+: b), with a 0 bit representing a and a 1 bit representing b.
However, consider a :+: b :+: c.
If the compiler treats this as (a :+: b) :+: c, then a is 00, b is 01, c is 1.
If it's a :+: (b :+: c), then a is 0, b is 10, c is 11.
The compiler's decision, even though (as I understand it) consistent for any given compile, could differ later.
The manual, at http://hackage.haskell.org/package/generic-deriving-1.6.3/docs/Generics-Deriving-Base.html\#g:9 states not to depend on the ordering, which the example does.
I think this is a valid concern, and the manual should be at least updated to discuss this problem.
Trac metadata
| Trac field | Value |
|---|---|
| Version | 7.8.2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Documentation |
| Test case | |
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| Blocking | |
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