[Haskell-beginners] How to structure a "speaking" fuzzy clock?

[Daniel Fischer]

On Sunday 16 January 2011 15:16:46, Kamil Stachowski wrote:
> Hello,
>
> I am writing a program, the main purpose of which is to print fuzzy time
> in a casual manner, e.g. "ten to eleven" rather than "10:52".
> Considering my (lack of) programming experience, I got pretty far with
> it with the help of D. Fischer and B. Yorgey; you can take a look at the
> results at https://github.com/caminoix/fuzzytime.
>
> However, I also have another purpose, and that is to better learn
> Haskell. I can't help the feeling that my solution might perhaps work,
> but is nevertheless ugly, naive or just un-Haskell.

From a quick glance: not too ugly.

In checkFTConf, you have a couple of guards

    | not (thing `elem` list)

which would read better using notElem,

    | thing `notElem` list

checkTimeOk isn't good, better

checkTimeOk
    = case break (== ':') time of
        (hh, _:mm) -> let h = read hh
                          m = read mm
                      in 0 <= h && h < 24 && 0 <= m && m < 60
        _ -> False

and to further guard against malformed input, you could also test
- not (null hh || null mm)
- all isDigit hh && all isDigit mm -- requires import Data,Char

The message for precision out of range should contain '<=' instead of '<'.

Are you sure you mean humane and not human?

In FuzzyTime, maybe it would be better to have the field fzLang be a new 
datatype Lang instead of a String.

In toFuzzyTime, get hour and min(ute) via break (and not per reverse . 
takeWhile (/= ':') . reverse), like in checkTimeOk

>
> Therefore, I would like to ask you how you would structure such a
> program.

Hmm, I would need to think about that. What I can say without thinking:

I'd use a new datatype for style and lang, and instead of checking the 
validity of the FuzzyTimeConf before converting to FuzzyTime, I'd write a 
conversion

foo :: FuzzyTimeConf -> Either ErrorMessage FuzzyTime

and do the checks during the conversion to not repeat work. 

> I'm not asking for an implementation, just a general idea, such
> as "a datatype to keep the fuzzy time, deriving Show with a set of
> separate functions for different languages".
>
> Thanks in advance!