[Haskell-beginners] Learn You a Haskell! I have a few questions!

[Bob Ippolito]

On Fri, Apr 18, 2014 at 8:12 AM, Gilberto Melfe <gilbertomelfe at gmail.com>wrote:

> Hi there to you all!
>
> I've been reading through the first chapter of "Learn You a Haskell" and
> I'd like to ask the community a few questions.
>
> Any help would be appreciated...
>
> -- ---------- ---------- ---------- ---------- ----------
>
> All the standard Prelude functions that throw out an error when fed the []!
>
> head
> maximum
> ...
>
> Are there safe versions anywhere, or do we have to define them ourselves?
>

Not so many that ship with GHC or Haskell Platform, but you can install
safe:
https://hackage.haskell.org/package/safe

Some of these you can work around, for example you can get a safe version
of `head` just by using Data.Maybe.listToMaybe


> -- ---------- ---------- ---------- ---------- ----------
>
> This one is really important!
>
> I understand that for the definition of product to work as it is:
>   product [] must be equal to 1
>
> But what if we want to add a product to something else???
> Shouldn't the result be Nothing?
> (I guess we would have to guard against this! But we must guard against
> the parameter of product being the empty list, anyway. Otherwise we risk
> adding 1 when there is nothing do multiply)
>
> (The same question arises with the functions and and or, and their boolean
> results, I think! Right?)
>

There's a precedent in mathematics for behaving like this. 0! and n^0 are
both equal to 1 for example. It sounds like perhaps you're trying to do
something strange with products of lists, and there might be a better way
but it's hard to suggest something without a concrete example.


>
> -- ---------- ---------- ---------- ---------- ----------
>
> -- ---------- Start Quote
>
> Names can't be enumerated. What comes after "John"? I don't know.
>
> -- ---------- End Quote
>
> "a" to "z" then "aa" to "zz" then "aaa" to "zzz" and so on! Is it to
> difficult or impossible to create a function that enumerates all possible
> strings?
>

This is not hard to implement, but you don't know which of those strings
are names.


> -- ---------- ---------- ---------- ---------- ----------
>
> -- ---------- Start Quote
>
> To make a list with all the numbers from 20 to 1, you can't just do
> [20..1], you have to do [20,19..1].
>
> -- ---------- End Quote
>
> Why is this? If the first was greater than the second it would just
> subtract! Right?
>

[a..b] is syntax sugar for enumFromTo and the definition of that function
just doesn't behave in that way. [a, b .. c] is syntax sugar for
enumFromThenTo which does. A reason for it to behave like this would be
that it's often desired to have the behavior that it does. Consider
enumerating every index in a list `xs` except for the first, you could
write this as `[1 .. length xs - 1]` with the current syntax, but that sort
of thing would yield surprising results if it sometimes went backwards.


> -- ---------- ---------- ---------- ---------- ----------
>
> -- ---------- Start Quote
>
> Watch out when using floating point numbers in ranges! Because they are
> not completely precise (by definition), their use in ranges can yield some
> pretty funky results.
>
> ghci> [0.1, 0.3 .. 1]
> [0.1,0.3,0.5,0.7,0.8999999999999999,1.0999999999999999]
>
> -- ---------- End Quote
>
> Can anyone explain me why it works for the first few values, and not
> "completely"?
>

It doesn't "work" for any of the values, it's just an artifact of how
they're rendered. 0.1 can't be exactly represented in binary floating
point, so the error compounds. Double probably shouldn't be enumerable in
the first place, but that's a decision we have to live with. The reason
that the end result is so surprising is that 1.0999999999999999 is less
than 1 + 0.1 and for whatever reason the way Enum is defined for Double
checks to see if the result is > to+(then-from) rather than <= to.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20140418/9df5faf1/attachment.html>