Data.Tree.Tree Show instance
Tue, 12 Aug 2003 12:30:27 +0200 (CEST)
On Mon, 11 Aug 2003, Tomasz Zielonka wrote:
> The Tree datatype in new Data.Tree module has a Show instance ...
I agree that Read/Show should normally come as a pair.
It is debatable whether a library should contain a Show instance
that makes it rather impossible to write the corresponding Read instance.
Even more so since no library user can later change this:
by the Haskell definition,
if you import a module that contains a data type definition,
then you also inherit all its instances from that module.
(Section 5.4 of the report, http://haskell.org/onlinereport/modules.html)
the workaround would be to move the `instance Show Tree'
into a separate library module.
On the other hand, it is my opinion that for a clean program design,
you should normally define your own `data' (or `newtype') types,
rather than use exisiting ones by `type' synonyms.
this may lead to somewhat larger program texts,
but you'll find that they are more readable, and extendable.
(Compare adding another component to a tuple, and to a record).
this also gives you the opportunity
to define class instances in any way you want.
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