# Fail/0 caused Internal server error

**URL:** <https://swi-prolog.discourse.group/t/fail-0-caused-internal-server-error/897>\
**Category:** Help!\
**Created:** [July 2, 2019, 4:35am UTC](https://swi-prolog.discourse.group/t/fail-0-caused-internal-server-error/897 "2019-07-02T04:35:02Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![HM\_MM](https://avatars.discourse-cdn.com/v4/letter/h/5daacb/32.png) [@HM\_MM](https://swi-prolog.discourse.group/u/HM_MM)\
**Post date:** [July 2, 2019, 4:35am UTC](https://swi-prolog.discourse.group/t/fail-0-caused-internal-server-error/897/1 "2019-07-02T04:35:02Z")

</div>

I’m using: SWI-Prolog version 7.6.4  
Why `fail/0` causes **Internal server error**?  
Thanks.

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<div class="post-metadata">

**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\
**Post date:** [July 2, 2019, 6:48am UTC](https://swi-prolog.discourse.group/t/fail-0-caused-internal-server-error/897/2 "2019-07-02T06:48:41Z")

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A bit of context please? Preferably something we can run, but at least some code and a description of the context it is used in.

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<div class="post-metadata">

**Author:** ![HM\_MM](https://avatars.discourse-cdn.com/v4/letter/h/5daacb/32.png) [@HM\_MM](https://swi-prolog.discourse.group/u/HM_MM)\
**Post date:** [July 2, 2019, 8:51am UTC](https://swi-prolog.discourse.group/t/fail-0-caused-internal-server-error/897/3 "2019-07-02T08:51:42Z")

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example:-  
**testinput.pl**

```prolog
:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- use_module(library(http/http_error)).
:- use_module(library(http/html_write)).
:- use_module(library(http/http_parameters)).
:- use_module(testoutput).
:- http_handler(root(test), web_form, []).

server(Port) :-
        http_server(http_dispatch, [port(Port)]).
web_form(_Request) :-
	reply_html_page(
            [title('e-TEST')],
	    [
	     form([action='/landing', method='POST'], [
		p([], [
		  label([for=name],'Please select: '),
		  input([name=test, type=radio, value="one"]),"TEST 1",
                  input([name=test, type=radio, value="two"]),"TEST 2"
		      ]),
		p([], input([name=submit, type=submit, value='Submit'], []))
	      ])]).

:- http_handler(root(landing), user_input, []).

user_input(Request):-
        http_parameters(Request,
                        [test(Input, [])
                        ]),
       format('Content-type: text/html~n~n', []),
       format('<table border=1>', []),
       format('<tr>', []),
       format('<td>', []),write("Your Input"),nl,format('</td>', []),
       format('<td>', []),format('<b>', []),write(Input),nl,format('</b>', []),format('</td>', []),
       format('</tr>', []),
       format('</table>', []),
       test_output(Input).

```

**testoutput.pl**

```prolog
%testoutput.pl
:- module(testoutput,
  [
    test_output/1
  ]).

test_output(X):-
    writeln(X),
    output.

output:-
    my_list(_, Output),
    writeln(Output),
    fail.

my_list(1, 'test 1').
my_list(2, 'test 2').
my_list(3, 'test 3').
my_list(4, 'test 4').
my_list(5, 'test 5').
my_list(6, 'test 6').
my_list(7, 'test 7').

```

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<div class="post-metadata">

**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\
**Post date:** [July 2, 2019, 10:53am UTC](https://swi-prolog.discourse.group/t/fail-0-caused-internal-server-error/897/4 "2019-07-02T10:53:03Z")

</div>

So, yes. Failure of a declared HTTP handler is considered an internal server error. Some exceptions are mapped to specific HTTP codes, but most also end up as internal sever error. What else would you expect?

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<div class="post-metadata">

**Author:** ![HM\_MM](https://avatars.discourse-cdn.com/v4/letter/h/5daacb/32.png) [@HM\_MM](https://swi-prolog.discourse.group/u/HM_MM)\
**Post date:** [July 3, 2019, 12:09am UTC](https://swi-prolog.discourse.group/t/fail-0-caused-internal-server-error/897/5 "2019-07-03T00:09:24Z")

</div>

any solution for this?

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<div class="post-metadata">

**Author:** ![michaelby](https://avatars.discourse-cdn.com/v4/letter/m/b3f665/32.png) [@michaelby](https://swi-prolog.discourse.group/u/michaelby)\
**Post date:** [July 3, 2019, 6:57am UTC](https://swi-prolog.discourse.group/t/fail-0-caused-internal-server-error/897/6 "2019-07-03T06:57:15Z")

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It’s not a problem. Don’t fail inside a handler if you don’t want an internal server error.

In this case, you could add a fact

```
output.

```

to the bottom of your `output/0` predicate to stop it from failing at the end.