Add -fextended-default-rules and -fmono-pat-binds
Add -fextended-deafult-rules (in response to Don Stewart's message below), and document them. Also doucument -fmono-pat-binds/-fno-mono-pat-binds, which has been in GHC a few weeks now. (The two are in one patch because the diffs were so close together that Darcs combined them.) Simon From: Donald Bruce Stewart [mailto:firstname.lastname@example.org] Sent: 07 August 2006 10:52 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?
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