Orphan Instances

Yitzchak Gale gale at sefer.org
Tue Aug 12 11:47:58 EDT 2008

<Moving this side point to the ghc users list...>

Ashley Yakely wrote:
>> What is an orphan instance, and why do we care about them?

Simon Peyton-Jones wrote:
> They are documented in the GHC manual
> http://www.haskell.org/ghc/docs/latest/html/users_guide/separate-compilation.html#orphan-modules

Thanks for the nice explanation there. Since you brought it
up, though, the explanation might be made even clearer with
two small changes:

1. The term "instance head" is used throughout the explanation,
beginning already with the second paragraph ("the head of the
instance..."). However, that term is only defined near the very end
'(the part after the "=>")'. For those who do not already know the
surprise ending, this makes the explanation read rather like a
mystery story.

2. I believe that the text "by setting a = Int" was meant to be
"by setting a = T".


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