Hi,
And now that we are at it... In the next page, 152 there is the
following instance definition, but no explanation is (I think) given of
what it means:
instance (JSON a) => JSON [a] where
until then all instance definitions where of the type
instance JSON Int where ...
How should I read that definition?
As a first approximation, read it as
"for all types a which are instances of JSON, the type [a] is also an
instance of JSON, with the following methods ..."
However, instance resolution in GHC is done without taking the context
(JSON a) into account, so for GHC it reads more like
"I will view all list types as instances of the JSON class, but if you try
to use the class instance for a list type where the element type is not an
instance of JSON, the programme will not compile. Nor will it compile if
you try to define another instance of JSON for any list type [e.g. String]
- at least, if you don't turn on some language extension(s)."
That is the cause of many puzzlements and problems.
thanks for this, and for the detailed explanation on my previous
question. I keep a frosty Canarian beer for you in the fridge for
whenever we meet... :-)
I think that I can now move on with the rest of the book. Cheers,
Ángel de Vicente
--
http://www.iac.es/galeria/angelv/
High Performance Computing Support PostDoc
Instituto de Astrofísica de Canarias
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