[erlang-questions] chained functions

Bengt Kleberg <>
Wed Jan 25 08:53:27 CET 2012


Greetings,

For things like this (seq/2 below) there is also lists:foldl/3 and
lists:foldr/3


bengt

On Tue, 2012-01-24 at 22:24 +0100, Gianfranco Alongi wrote:
> What an error!
> Should be:
> 
> If all your functions F_1 ... F_K have the type
> 
>  F(Input) ->  {ok,Result} | {error,Reason}
> 
> then I would probably write a small sequencer as
> 
> seq(X,[]) ->
>    X;
> seq({ok,Result},[ F | T ]) ->
>   seq(F(Res),T);
> seq({error,Reason},_) ->
>   {error,Reason}.
> 
> And call it like
> 
> seq({ok,Initial},[ fun F_1, ..., fun F_K ])
> 
> /G
> 
> (And that's why you don't write your emails in a haste before bed!!!)
> 
> On Tue, Jan 24, 2012 at 10:17 PM, Gianfranco Alongi
> <> wrote:
> > If all your functions F_1 ... F_K have the type
> >
> > F({ok,Result} | {error,Reason} ) ->  {ok,Result} | {error,Reason}
> >
> > then I would probably write a small sequencer as
> >
> > seq({ok,Result},[ F | T ]) ->
> >  seq(F(Res),T);
> > seq(Error,T) ->
> >  Error.
> >
> > And call it like
> >
> > seq({ok,Initial},[ fun F_1, ..., fun F_K ])
> >
> > I always try to tag values, so you can use it to determine flow control.
> >
> > Cheers
> > G
> >
> > 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
> >> _______________________________________________
> >> erlang-questions mailing list
> >> 
> >> http://erlang.org/mailman/listinfo/erlang-questions
> _______________________________________________
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> 
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