I just realized that the thread of this conversation was not fully finished. For Show/Read, there are two flex points: (1) Should `show (mkNF 2)` output "mkNF 2" or "UnsafeNF 2"? I have a preference for the former but then we need to handwrite the instance. We could also change the definition of NF to just name its constructor NF and export a helper function unsafeMkNF (2) Should `read "mkNF 2"` execute rnf on the result of the inner Read instance? If the answer is no, we can use the default; if the answer is yes, we want an instance like: instance (NFData a, Read a) => Read (NF a) where readsPrec = parens . prec 10 $ do Ident "makeNF" <- lexP m <- step readPrec return (m `deepseq` UnsafeNF m) I lean towards a safe by default API, with an unsafeReadNF, but maybe if default Read instances are not lazy, we should be OK (e.g. (read $ show (repeat 1)) :: [Int] hangs; it isn't identity). Edward Excerpts from Roman Cheplyaka's message of 2015-04-22 13:28:10 -0700:
On 22/04/15 21:54, Edward Z. Yang wrote:
But it is an interesting question whether or not 'UnsafeNF' should be used, since the value read in is known to be in normal form.
Is it?
newtype X = X Int deriving Show instance Read X where readsPrec n = map (first $ X . trace "eval") . readsPrec n
length (read "[1,2,3]" :: [X]) 3 read "[1,2,3]" :: [X] [X eval 1,X eval 2,X eval 3]
Or did you mean something else?
Excerpts from Henning Thielemann's message of 2015-04-22 19:49:56 +0100:
On Wed, 22 Apr 2015, Dan Burton wrote:
A hand-written read makes more sense to me in this case: read = makeNF . read show = show . getNF
Show and Read instances should process Strings representing Haskell code, and I guess, Haskell code with the same type as the represented value. Thus the NF should be part of the formatted value.
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