Hello Wolfgang,

you are quite right, most loops can easily be
transformed by using map, filter, fold_left or
fold_right. I just tend to forget about it or find it
uneasy if the problem is more complex. It may be just
a matter of habit.

Regards,

Martin

--- Wolfgang Lux <wlux@uni-muenster.de> schrieb:

> Pal-Kristian Engstad wrote:
> 
> > The author argues that "Writing loops with
> tail-recursive function 
> > calls is
> > the equivalent of writing them with goto’s." and
> gives an example that 
> > I've
> > rewritten from Scheme-ish into OCaml-ish:
> >
> > let myfunc l =
> >   let rec loop rest result =
> >     match rest with
> >       | [] -> List.rev result
> >       | x::xs ->
> > 	  if xpred x then
> > 	    let y = verbose_code_using_x x in
> > 	      if ypred y then
> > 		let z = verbose_code_using_y y in
> > 		  loop xs (z_expression :: result)
> > 	      else
> > 		loop xs result
> > 	  else
> > 	    loop xs result
> >   in
> >     loop l []
> > ;;
> >
> > Obviously, one would like to refactor this into
> HOF, but in this 
> > situation it
> > is hard to see how one should do it.
> 
> Oh come on! IMHO, most loops can easily be
> transformed by using map, 
> filter,
> fold_left, and fold_right. The example given is no
> exception. The loop
> essentially applies some complex code to every
> element of the list and
> includes the result in the output list only if a
> condition is satisfied
> that is computed along with result. This is always
> the structure of a 
> fold.
> (If the condition were independent from the result
> one could combine a 
> filter
> and a map.) Thus, a straight forward rewrite of the
> loop is:
> 
>    let myfunc l =
>      let f x result =
>            if xpred then
>              let y = verbose_code_using_x x in
>                if ypred y then
>                  let z = verbose_code_using_y y in
>                    z_expression :: result
>                else
>                  result
>            else
>              result
>      in
>        fold_right f l []
> 
> Furthermore, I would turn the two if expressions
> into local functions
> 
>    let myfunc l =
>      let g y result =
>            if ypred y then
>              let z = verbose_code_using_y y in
> z_expression :: result
>            else
>              result
>      in let f x result =
>           if xpred x then
>             let y = verbose_code_using_x x in g y
> result
>           else
>             result
>      in fold_right f l []
> 
> Regards
> Wolfgang
> 
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