[Haskell-cafe] Trying to write 'safeFromInteger'

[Henning Thielemann]

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-Maybe-HT.html#v%3AtoMaybe