[erlang-questions] Orthogonality and Principle of least surprise Was: chained functions

Jakob Praher <>
Wed Feb 1 09:05:10 CET 2012


Hi Erlangers,

I think people should pay attention to issues faced by experienced newcomers in exercising Erlang programming. Matz from Ruby calls a ver important design principle, the principle of least surprise. 

No matter that the following may be a syntactic surface issue, but designating function objects is neither orthogonal nor does follow the expectations of the programmer. Therefore it also makes understanding code harder.

if I can write:
     module:fun().
I should be also able to write
     F = module:fun, F().
and not having to write
     F = fun module:fun/0, F().
.

I am happy to learn why module:fun is not an expression.

Best,
Jakob


Am Dienstag, 31. Januar 2012 14:07 CET, "Jakob Praher" <> schrieb: 
 
>  
> As I tried out the code below I learned that tuples of module and function atoms evalute to functions, not atoms by themselves.
> If one can apply arguments to a tuple, why not to an atom if this would be denote a function in the current environment? 
> 
> {R, V} = apply_each([{m1,z},{m2,y},{m3,x}], Arg).
> if R = ok -> ...
> 
> Thanks,
> Jakob
> 
> 
> Am Mittwoch, 25. Januar 2012 14:29 CET, "Jakob Praher" <> schrieb: 
>  
> > 
> > One could also compose a sequence of functions much like in combinator parsing, and fail at the first event returning the error status, otherwise continue to call apply the functions to the result.
> > 
> > apply_each([F|FS],X) -> 
> >       {S, V} = F(X), 
> >       if S = error  -> 
> >             {S, V}
> >          S = ok -> 
> >             apply_each(FS, V);
> >       end
> >       .
> > 
> > {R, V} = apply_each([z,y,x], Arg).
> > if R = ok -> ...
> > 
> > I have not much practice in Erlang (though I have some practice in Prolog), so this might not be 100 % correct.
> > 
> > Cheers,
> > Jakob
> > 
> > Am Mittwoch, 25. Januar 2012 13:16 CET, Joe Armstrong <> schrieb: 
> >  
> > > I think you are asking the wrong question. If a function returns
> > > {ok,Val} | {error,Reason}
> > > then I think to myself "the caller of this function expects things to
> > > go wrong and is
> > > committed to handling *both* return values.
> > > 
> > > So they would *never* write w(x(y(z(....))) because this does not cater for both
> > > return values.
> > > 
> > > With the "let it crash" philosophy one would make w,x,y, .. return a
> > > value OR call exit(...).
> > > With this convention things do nest in the "happy" case without using
> > > a staircase.
> > > 
> > > At the top level try or catch is used to catch the error.
> > > 
> > > Alternatively you could say
> > > 
> > >   ok({ok,X}) -> X;
> > >   ok{error,E}) -> exit(E).
> > > 
> > > and then
> > > 
> > >    w(ok(x(ok(y(ok(z(X))))))
> > > 
> > > Not pretty but it does the job
> > > 
> > > /Joe
> > > 
> > > 
> > > On Tue, Jan 24, 2012 at 8:31 PM, Reynaldo Baquerizo
> > > <> wrote:
> > > >
> > > > A friend of mine asked:
> > > >
> > > > ##
> > > > If you have functions that return {ok, Result} | {error, Reason}
> > > > how do you chained them? So that you have:
> > > >
> > > > w(x(y(z(...))))
> > > >
> > > > without building a staircasing. Something that would be done in Haskell
> > > > with monads.
> > > > ##
> > > >
> > > > I would probably go for:
> > > >
> > > > x({ok, Value}) ->
> > > >  NewValue = <do something with Value>,
> > > >  {ok, NewValue};
> > > > x({error, Reason}) ->
> > > >  {error, Reason}.
> > > >
> > > > in each function
> > > >
> > > > which brings me other question, when do you tag return values?
> > > >
> > > > I tend to only use tagged return values with impure functions, were an
> > > > error is more likely due to side effects.
> > > >
> > > > --
> > > > Reynaldo
> > > > _______________________________________________
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