Re: [Haskell-cafe] Avoiding undecidables

Am Samstag 05 Dezember 2009 21:51:26 schrieb Michael Snoyman:

On Sat, Dec 5, 2009 at 10:33 PM, José Iborra wrote:

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Since you have already decided MPTCs are ok, in my opinion undecidable instances are fine here. But I realize this is not the usual stance, so I might be wrong.

As far as I understand, all UndecidableInstances does is tell the compiler "Maybe instance checking won't terminate, try anyway", so they're dangerous *only during compilation*, once things compile, everything's dandy.

Sounds reasonable to me. I'm waiting for the boogey man to jump out though

and explain why undecidables here will get me gored to death by a raptor [1].

They won't. They might suck you into a black hole, but raptors are very specialised.

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instance Convert Integer Double where convert = Just . fromIntegral instance TotalConvert Double -- that's all

instance TotalConvert Integer Double

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Interestng approach. The current approach is *almost* the same, just leaves

off the default definition. I'd be wary of putting in a definition like that; although it saves a line of typing, it let's a partial function get in which could cause trouble for unsuspecting users. But it is a possibility.

But only if they declare misbehaved instances of TotalConvert. Whenever a reasonable instance TotalConvert x y exists, the Convert instance should satisfy the demands of the default definition.

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Cheers, pepe

[1] http://xkcd.com/292/