Here is a simple Maplet example,
T:=proc()
global f,g,m;
uses Maplets, Maplets:-Elements;
f:=(a,b,c)->is(min(a+b-c,a-b+c,-a+b+c)>0);
g:=proc() uses Maplets:-Tools;
if f(parse(Get('a')),parse(Get('b')),parse(Get('c'))) then
   Set('d'=parse(Get('a'))+parse(Get('b'))+parse(Get('c')));
   Set('L'('visible')=false) 
else Set('d'=""); Set('L'('visible')=true) 
fi end; 
m := Maplet(["Enter the side lengths:",
    [TextField['a']('value'=`if`(nargs>0,args[1],3), 10),
	 TextField['b']('value'=`if`(nargs>1,args[2],4), 10),
	 TextField['c']('value'=`if`(nargs>2,args[3],5), 10)],
    Label['L']("A triangle with these sides doesn't exist.\n", 
         'visible'='false'),
    Button("Perimeter",Evaluate('function'="g")),
    TextField['d']()]);
Display(m) end:

T();

                

T(1,2,3);
    
                

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