More defaulting

[Simon Peyton-Jones]

You caught me at a succeptible moment. I've implemented
-fextended-default-rules as you suggest.

Simon

| -----Original Message-----
| From: Donald Bruce Stewart [mailto:dons at cse.unsw.edu.au]
| Sent: 07 August 2006 10:52
| To: cvs-ghc at haskell.org
| Cc: Simon Peyton-Jones
| Subject: More defaulting
| 
| While we're thinking about defaulting, I have a question..
| 
| ghci uses an extended defaulting system, to allow things like:
|         Prelude> reverse []
|         []
| to work, and to have the right instance of Show found. The manual
says:
| 
|     "..it is tiresome for the user to have to specify the type, so
GHCi extends
|     Haskell's type-defaulting rules (Section 4.3.4 of the Haskell 98
Report
|     (Revised)) as follows. If the expression yields a set of type
constraints
|     that are all from standard classes (Num, Eq etc.), and at least
one is
|     either a numeric class or the Show, Eq, or Ord class, GHCi will
try to use
|     one of the default types, just as described in the Report. The
standard
|     defaulting rules require that one of the classes is numeric; the
difference
|     here is that defaulting is also triggered at least one is Show,
Eq, or Ord."
| 
| Currently, there is no way to get at this "extended" defaulting for
compiled
| modules. However, I have a use case for in fact doing this.
| 
| With runtime evaluated Haskell, embedding 'interpreters' (over
hs-plugins) is
| easy. lambdabot, for example, implements a sandboxed haskell eval
system. But
| it doesn't have access to the defaulting mechanism of ghci, so we
have:
| 
|     dons:: > reverse []
|     lambdabot:: Add a type signature
|     dons:: > reverse [] :: [()]
|     lambdabot:: []
| 
| Which is annoying -- newbies wonder why they have to add these extra
| constraints to get a Show instance.
| 
| I'm wondering, since the extended defaulting mechanisms are already
implemented,
| could they be made available to compiled modules as well, perhaps
using a flag,
|  -fextended-defaulting? Are there any technical reasons not to do
this?
| 
| -- Don