On Fri, Jul 13, 2012 at 06:02:02PM +0200, Thomas Bach wrote:
Hi Brent,
thanks for your suggestions. I got further suggestions as a private e-mail and finally came up with the following solution – suggestions welcome:
Looks good!
The reducer still throws an error when piping in an empty newline. But, I'm not sure, what a proper solution for this could be.
The problem is your 'summation' function: summation :: Num b => [(a, b)] -> (a, b) In fact, it is impossible to implement something with this type which works for all inputs. If you get the empty list as input, there is no way to make up a value of type 'a' in the output tuple. Now, if you happen to know a more concrete type for 'a' then you can do it by using some sort of default value. For example, in your case it looks like 'a' will actually be TL.Text. So you could write summation :: Num b => [(TL.Text, b)] -> (TL.Text, b) by doing something like returning the empty string along with 0 in the case that the input list is empty. In this particular case it is probably not that big of a deal. It is just good practice to try to always write *total* functions, that is, functions which give a sensible result (i.e. do not crash) for every possible input. This means doing things like (1) make sure you have covered every possible case when pattern-matching; (2) do not use functions which can crash like 'head' or 'tail' (use pattern-matching instead).
On Tue, Jul 10, 2012 at 10:19:48AM -0400, Brent Yorgey wrote:
On Thu, Jul 05, 2012 at 03:50:32PM +0200, Thomas Bach wrote:
Instead of using a chain of ($), it's generally considered better style to use a chain of (.) with a single $ at the end, like
I often find myself trying different combinations of ($), (.) or putting things in brackets, when the compiler throws an error on me. Without really knowing why things sometimes work out and sometimes don't. I find this part especially confusing!
This can be confusing at first. The solution is (1) make sure you understand how parsing/operator precedence work, and (2) think hard about the types of operators like ($) and (.), and what any type errors are telling you. You really will not learn much by just randomly trying to insert ($), (.) or parentheses. -Brent