A Tail Call, that runs (not) if different numeric values are being used as parameter

On Sat, 13 Apr 2019 at 06:06, Andreas Mixich <mixich.andreas at gmail.com> wrote: > Hi, > > as far as I have understood, a tail call function must end with the call > to itself, after it has calculated all the values and, upon recursive call, > no operation must be open in the calling instance of the function. This is > why the following case confuses me. > > The function, as shown, produces a stack overflow and I get the hint to do > tail recursion. However, if I swap just the expressions (after 'then' and > 'else') in lines 13 and 14, the function computes. Now I would have > thought, that it would break, because I recurse within another function, > that is still open ('concat()') and thus must return to the calling > function after recursion, but, interestingly, the swap of the expressions > after the 'then' and 'else' statements on lines 13 and 14 do not error out > with a stack overflow, in spite 'concat()' still being open. Both cases > produce an integer. Also, advice would be welcome, on how I can solve this. > Thank you very much. > > 1 declare function local:word-wrap( > 2 $string as xs:string, > 3 $width as xs:integer) > 4 > 5 { > 6 let $t := normalize-space($string) > 7 return > 8 if (string-length($t) < $width or string-length($t) = $width) > 9 then $t > 10 else > 11 let $idx := > 12 if (matches($t, "\s")) > 13 then functx:index-of-string-last(substring($t, $width), " ") > 14 else $width > 15 return > 16 concat( > 17 string:trim-right(substring($t, 1, $idx)), > 18 out:nl(), > 19 local:word-wrap(string:trim-left(substring($t, $idx)),$width) > 20 ) > 21 }; > 22 > 23 local:word-wrap("This is a very long line of text. It goes well beyond > 80 (eighty) characters in width. Therefore we need a word wrap function, > which we test in here. Lorem ipsum dolor sit amet, consectetur adipisicing > elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut > enim ad minim ve iam, quis nostrud exercitation ullamco laboris nisi ut > aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in > voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint > occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit > anim id est laborum.",80) > > -- > Minden jót, all the best, Alles Gute, > Andreas Mixich >

> I wonder if a simple regex

> Though, I still wonder, why the exchange of the two expressions did what > it did.

On Sat, Apr 13, 2019 at 11:55 AM Andy Bunce <bunce.andy at gmail.com> wrote: > > Hi Andreas, > > Ignoring for a moment the mysteries of tail recursion, I wonder if a simple regex can give you the wrapping you are looking for. See [1] > > declare function local:word-wrap($string,$width){ > $string=>normalize-space()=>concat(" ")=>replace(``[(.{0,`{ $width }`}) ]``,'$1&#xA;') > }; > "This is a very long line of text. It goes well beyond 80 (eighty) characters in width. Therefore we need a word wrap function, which we test in here. Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim ve iam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum." > =>util:replicate(10000)=>string-join("") > > =>local:word-wrap(80) > > /Andy > [1] https://stackoverflow.com/a/3892909/3210344 > > On Sat, 13 Apr 2019 at 06:06, Andreas Mixich <mixich.andreas at gmail.com> wrote: >> >> Hi, >> >> as far as I have understood, a tail call function must end with the call to itself, after it has calculated all the values and, upon recursive call, no operation must be open in the calling instance of the function. This is why the following case confuses me. >> >> The function, as shown, produces a stack overflow and I get the hint to do tail recursion. However, if I swap just the expressions (after 'then' and 'else') in lines 13 and 14, the function computes. Now I would have thought, that it would break, because I recurse within another function, that is still open ('concat()') and thus must return to the calling function after recursion, but, interestingly, the swap of the expressions after the 'then' and 'else' statements on lines 13 and 14 do not error out with a stack overflow, in spite 'concat()' still being open. Both cases produce an integer. Also, advice would be welcome, on how I can solve this. Thank you very much. >> >> 1 declare function local:word-wrap( >> 2 $string as xs:string, >> 3 $width as xs:integer) >> 4 >> 5 { >> 6 let $t := normalize-space($string) >> 7 return >> 8 if (string-length($t) < $width or string-length($t) = $width) >> 9 then $t >> 10 else >> 11 let $idx := >> 12 if (matches($t, "\s")) >> 13 then functx:index-of-string-last(substring($t, $width), " ") >> 14 else $width >> 15 return >> 16 concat( >> 17 string:trim-right(substring($t, 1, $idx)), >> 18 out:nl(), >> 19 local:word-wrap(string:trim-left(substring($t, $idx)),$width) >> 20 ) >> 21 }; >> 22 >> 23 local:word-wrap("This is a very long line of text. It goes well beyond 80 (eighty) characters in width. Therefore we need a word wrap function, which we test in here. Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim ve iam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.",80) >> >> -- >> Minden jót, all the best, Alles Gute, >> Andreas Mixich