Max Rabkin writes:

The problem with your code is that the type of maxBound is unspecified. You need (maxBound `asTypeOf` i), or enable ScopedTypeVariables and use (maxBound :: a) (I think).

I tried doing the (maxBound :: a) thing, but got another confusing error: safeFromInteger :: (Num a, Integral a, Bounded a) => Integer -> Maybe a safeFromInteger i = if i > (toInteger (maxBound :: a)) then Nothing else Just (fromInteger i) # ghci -XScopedTypeVariables Test.hs Test.hs:3:20: Could not deduce (Bounded a1) from the context () arising from a use of `maxBound' at TestIntegerBounds.hs:3:20-27 Possible fix: add (Bounded a1) to the context of an expression type signature In the first argument of `toInteger', namely `(maxBound :: a)' In the second argument of `(>)', namely `(toInteger (maxBound :: a))' In the expression: i > (toInteger (maxBound :: a))

On Tue, 7 Apr 2009, Max Rabkin wrote:

The problem with your code is that the type of maxBound is unspecified. You need (maxBound `asTypeOf` i), or enable ScopedTypeVariables and use (maxBound :: a) (I think).

i is Integer, so asTypeOf is not so easy to apply. I propose to write safeFromIntegerAux :: (Num a, Integral a) => a -> Integer -> Maybe a safeFromIntegerAux mb i =    if i > toInteger mb      then Nothing      else Just (fromInteger i) safeFromInteger = safeFromIntegerAux maxBound Btw. this is another instance where my beloved toMaybe function is useful. :-) http://hackage.haskell.org/packages/archive/utility-ht/0.0.4/doc/html/Data-M...

Hello Henning, Wednesday, April 8, 2009, 1:43:31 AM, you wrote:

i is Integer, so asTypeOf is not so easy to apply. I propose to write

safeFromInteger i = let mb = maxBound in if i > (toInteger mb) then Nothing else Just (fromInteger i `asTypeOf` mb) or you can do it opposite way: safeFromInteger i = let res = fromInteger i in if i > (toInteger (maxBound `asTypeOf` res)) then Nothing else Just res -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

That seems a really weird way to write it! Who decided all auxiliary functions should be called go? (I think I'm blaming dons) - why not: sffi :: (Integral a,Num a) => Integer -> Maybe a sffi n | toInteger n2 == n = Just n2 | otherwise = Nothing where n2 = fromInteger n No need for auxiliary function definitions. Thanks Neil

On Wed, Apr 8, 2009 at 9:26 AM, david48 wrote:

On Wed, Apr 8, 2009 at 2:55 PM, Neil Mitchell wrote:

...

That seems a really weird way to write it! Who decided all auxiliary functions should be called go? (I think I'm blaming dons) - why not:

sffi :: (Integral a,Num a) => Integer -> Maybe a sffi n | toInteger n2 == n = Just n2        | otherwise = Nothing     where n2 = fromInteger n

I know I was too lazy to clean it up :-P ( I also blame Dons for 'go' )

I think the Common Lisp community tends to use 'foo-aux' instead of 'go' for these sort of axillary functions. But, then in Haskell we can't use hyphen as an identify character and underscore is not popular. For this reason I started using fooAux in Haskell, but after learning that a single quote is valid identifier character I started using foo'. Other than using go and foo', what do people use in Haskell? Jason

On Wed, Apr 08, 2009 at 09:38:25AM -0700, Jason Dagit wrote:

Other than using go and foo', what do people use in Haskell?

I frequently use f and g. -- Antti-Juhani Kaijanaho, Jyväskylä, Finland http://antti-juhani.kaijanaho.fi/newblog/ http://www.flickr.com/photos/antti-juhani/

...

...

That seems a really weird way to write it! Who decided all auxiliary functions should be called go? (I think I'm blaming dons) - why not:

sffi :: (Integral a,Num a) => Integer -> Maybe a sffi n | toInteger n2 == n = Just n2        | otherwise = Nothing     where n2 = fromInteger n

I know I was too lazy to clean it up :-P ( I also blame Dons for 'go' )

I think the Common Lisp community tends to use 'foo-aux' instead of 'go' for these sort of axillary functions.  But, then in Haskell we can't use hyphen as an identify character and underscore is not popular.  For this reason I started using fooAux in Haskell, but after learning that a single quote is valid identifier character I started using foo'.

Other than using go and foo', what do people use in Haskell?

I use f, if I need several auxiliary functions I start at f and work my way up alphabetically. I tend to go back to f2 if I go past h. Be grateful you don't have to maintain my code :-) Thanks Neil PS. Here is some code from the filepath library I wrote, illustrating how fantastic my naming scheme looks. (I think if I was writing it today I'd have used a list comprehension for validChars, eliminating f.) makeValid path = joinDrive drv $ validElements $ validChars pth where (drv,pth) = splitDrive path validChars x = map f x f x | x `elem` badCharacters = '_' | otherwise = x validElements x = joinPath $ map g $ splitPath x g x = h (reverse b) ++ reverse a where (a,b) = span isPathSeparator $ reverse x h x = if map toUpper a `elem` badElements then a ++ "_" <.> b else x where (a,b) = splitExtensions x

On Wed, 8 Apr 2009, Neil Mitchell wrote:

That seems a really weird way to write it! Who decided all auxiliary functions should be called go? (I think I'm blaming dons) - why not:

sffi :: (Integral a,Num a) => Integer -> Maybe a sffi n | toInteger n2 == n = Just n2 | otherwise = Nothing where n2 = fromInteger n

I think it is a bug, that 'fromInteger' is defined for numbers outside the range of the target type. Why is it necessary to have, say fromInteger 1000 == (232 :: Word8) ? I would not rely on this behaviour and safeFromInteger should also work, when the fromInteger method yields undefined for some Integer input.