[erlang-questions] nand problem

[Joe Armstrong]

On Mon, Jan 26, 2015 at 10:53 PM, Roelof Wobben <r.wobben@REDACTED> wrote:
> Joe Armstrong schreef op 26-1-2015 om 22:35:
>
>> On Mon, Jan 26, 2015 at 6:26 PM, Hugo Mills <hugo@REDACTED> wrote:
>>>
>>> On Mon, Jan 26, 2015 at 05:21:42PM +0100, Roelof Wobben wrote:
>>> [snip]
>>>>
>>>> b_and(true, true) ->
>>>>    true;
>>>>
>>>> b_and(true, false) ->
>>>>    false;
>>>>
>>>> b_and(false, true) ->
>>>>    false;
>>>>
>>>> b_and(false, false) ->
>>>>    false.
>>>
>>>     You could make this shorter and possibly easier to read with
>>>
>>> b_and(true, true) ->
>>>      true;
>>> b_and(_, _) ->
>>>      false.
>>
>> At which point I feel compelled to ask
>> do you really really really want
>>
>> b_and(1, any_old_stupid_argument) to evaluate to false?
>>
>> by explicitly requiring the arguments of b_and to be true or false
>> *and nothing else* you ensure that a program calling b_and with stupid
>> arguments will crash *as soon as possible* which will make debugging
>> easier. It also enables a whole bundle of compiler optimisations -
>> making arguments more general than they need to be makes analysis more
>> difficult
>> and compiled code more general. The more you tell the compiler, then
>> better code it can produce.
>>
>> Yes the program is shorter (good) - but it's not totally correct (bad).
>>
>> This is not important for small programs, but for huge programs making
>> every
>> function totally correct is essential - correct well typed code can be
>> written
>> stuck in a library and forgotten forever - code that does silly things
>> with silly
>> arguments is not good programming practise.
>>
>> b_not should be defined over boolean arguments - any non boolean
>> argument should raise an exception *immediately* anything else
>> violates
>> the principle of least astonishment.
>>
>> Cheers
>>
>> /Joe
>>
>>
>>
>>
>>
>>
>>>     i.e. define the (one) special case, and then just say that
>>> everything else evaluates to false. You can do something similar with
>>> b_or.
>>>
>>>     Hugo.
>>>
>>> --
>>> Hugo Mills             | Two things came out of Berkeley in the 1960s:
>>> LSD
>>> hugo@REDACTED carfax.org.uk | and Unix. This is not a coincidence.
>>> http://carfax.org.uk/  |
>>> PGP: 65E74AC0          |
>>>
>>> _______________________________________________
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>>>
>
>
> Oke, So for you the long version is well. Write every case so when someone
> uses something else then true or false will raise a error.
> Or do I misunderstood you ?

That's write - correctness is preferable to brevity

/Joe

>
> Roelof
>
>
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