On 24 Feb 2009, at 08:21, 7stud wrote:

Hi,

I just bought Real World Haskell, and I'm trying to get the ghc compiler set up. The installer for the latest version of ghc, 6.10.1, is for Leopard(osx 10.5). What should I install for osx 10.4.11(Tiger)?

Try using the version in MacPorts.

Also, a question about "Beware of enumerating floating point numbers" on p. 11. The book doesn't explain what it means to enumerate floating point numbers without a step, so when it says to beware of the non-intuitive behavior of:

ghci> [1.0..1.8]

how would a reader know what the intuitive behavior is? Is the step .1, .01, .001? Is there a default step? Who knows?

I'd say that if you can't figure out what it should do, that's pretty non intuitive, no? Bob

Am Dienstag, 24. Februar 2009 08:21 schrieb 7stud:

Hi,

Also, a question about "Beware of enumerating floating point numbers" on p. 11. The book doesn't explain what it means to enumerate floating point numbers without a step, so when it says to beware of the non-intuitive behavior of:

ghci> [1.0..1.8]

how would a reader know what the intuitive behavior is? Is the step .1, .01, .001? Is there a default step? Who knows?

Experienced Haskellers know :-) The default step is 1, and Prelude> [1.0 .. 1.8] [1.0,2.0] , which is what is meant by "the non-intuitive behaviour". Of course, a learner won't know, so a request to the authors: elaborate that in the second edition. The fact behind it is that [a .. b] is syntactic sugar for enumFromTo a b , enumFromTo is a method in the class Enum. As the name suggests, this class is for types where you can enumerate the elements, every element - except boundary elements - should have a well defined predecessor and successor. So, enumFromTo a b is in these cases the sequence of elements c with index a <= index c <= index b, the indices increasing by 1. This doesn't make much sense for floating point types or Rational, of course. However, arithmetic sequences do make a lot of sense for these types, so they are made instances of the class Enum, too. Since their elements don't have a natural indexing, these instances are by necessity somewhat broken. The enumFrom and enumFromTo methods proceed in steps of 1, which in absence of a reasonable successor function is probably the best choice. However, enumFromTo a b doesn't stop when the value exceeds b, but when it exceeds b+1/2. The behaviour is defined in section 6.3.4 of the Haskell report.

On p. 10 there is an error. Below the last code example on the page, it says:

"In the latter case, the list is quite sensibly missing the endpoint of the enumeration, because it isn't an element of the series we defined."

But the last line of the code example is this:

ghci> [10,9..1] [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

which quite clearly includes both endpoints.

They probably meant to write [10,8 .. 1].