Make an interpreter for yes!

Question

Yes is a stack-based language that has a few space-separated instructions:

yes: Push 1 to the stack
no: Push 0 to the stack
what: Push the input to the stack (input is taken at the start of program execution and is the same for the whole execution)
sure: Increment the last item in the stack
nah: Decrement the last item in the stack
really: If the last stack item is a number, replace it with its Unicode character. If it is a letter, replace it with its Unicode char code.
oh: convert the stack to strings and concatenate it, and push that to the stack.
nope: remove the first stack item
yep: remove the last stack item

The stack's last element is always outputted at program end. All non-alphanumeric and non-space characters are ignored. All code is lowercase. Example programs:

yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
oh

prints Hello, World!.

what

prints the input (cat program.)

no nah

prints -1.

no really

prints the NUL character (U+0000)

what 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
oh

prints the input and an underscore.

yes no nope

prints 0

yes no yep

prints 1.

You must write an interpreter in as few bytes as possible. Here's a JS implementation (not well golfed!):

function yes(code, input){
	var stack = [];
	var functions = {
		"yes": "stack.push(1)",
		"no": "stack.push(0)",
		"what": "stack.push(input)",
		"sure": "stack[stack.length - 1] ++",
		"nah": "stack[stack.length - 1] --",
		"really": "stack[stack.length - 1] = (typeof lastItem === 'number' ? String.fromCharCode(lastItem) : lastItem.charCodeAt())",
		"oh": "stack.push(stack.reduce((x, y)=>''+x+y))",
		"nope": "stack.shift()",
		"yep": "stack.pop()"
	};
	code.replace(/[^a-z ]/g, "").split(" ").map(x=>(lastItem = stack[stack.length - 1],eval(functions[x])));
	return stack[stack.length - 1];
}

textarea{
  display: block;
}

Code: <textarea id = "code"></textarea>
Input: <textarea id = "input"></textarea>
<button onclick = "output.value = yes(code.value, input.value)">Run</button>
<textarea id = "output"></textarea>

See also my JS answer below.

You may assume that all integers involved will be less than or equal to 126, that really will never be run with a multi-char string on top of the stack, and that the stack will never be longer than 100 elements.

Edge cases

• yes yes oh prints 11.
• Intput may be a string or number.
• The code can contain any chars. Ones not matching [a-z ] should be ignored.

Answer 1

JavaScript (ES6), 218 215 204 203 bytes

Takes the program string s and the input i in currying syntax (s)(i).

s=>i=>s.replace(/\w+/g,S=>(c=eval("[P()];P()+1;[s.shift()];1;0;s=[s.join``];P()-1;i;P()[0]?k.charCodeAt():String.fromCharCode(k)".split`;`[parseInt(S,35)%156%9])).map||s.push(c),s=[],P=_=>k=s.pop())&&P()

How?

We use the perfect hash function parseInt(S, 35) % 156 % 9 to convert the instruction S into an index in 0…8 and use this index to select the JS code to be executed:

instruction | base 35 -> dec. | % 156 | % 9 | JS code
------------+-----------------+-------+-----+---------------------------------------------
"yes"       |           42168 |    48 |   3 | 1
"no"        |             829 |    49 |   4 | 0
"what"      |         1393204 |   124 |   7 | i
"sure"      |         1238209 |    37 |   1 | P()+1
"nah"       |           28542 |   150 |   6 | P()-1
"really"    |      1439554619 |    35 |   8 | P()[0]?k.charCodeAt():String.fromCharCode(k)
"oh"        |             857 |    77 |   5 | s=[s.join``]
"nope"      |         1016414 |    74 |   2 | [s.shift()]
"yep"       |           42165 |    45 |   0 | [P()]

The P function pops the last item off the stack s and loads it into k.

We prevent the result of some instructions from being pushed back onto the stack by testing if the .map() method is defined, that is, if the result is an array. The code for oh is returning an array by design and we force nope and yep to return arrays as well. Hence the syntax:

(c = eval("[code0];code1;...".split`;`[index])).map || s.push(c)

Test cases

let f =

s=>i=>s.replace(/\w+/g,S=>(c=eval("[P()];P()+1;[s.shift()];1;0;s=[s.join``];P()-1;i;P()[0]?k.charCodeAt():String.fromCharCode(k)".split`;`[parseInt(S,35)%156%9])).map||s.push(c),s=[],P=_=>k=s.pop())&&P()

console.log(f(`yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
oh`)())

console.log(f(`what`)(1234))

console.log(JSON.stringify(f(`no really`)()))

console.log(f(`what 
yes sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure sure really 
oh`)('code golf'))

console.log(f(`yes no nope`)())

console.log(f(`yes no yep`)())

Answer 2

Röda, 256 bytes

f c,n{s=[];(c/`\W|_`)()|{|m|s+=#m-2 if[m=~"yes|no"];s[-1]+=#m*2-7 if[m=~"sure|nah"];s+=n if[m="what"];s=s[#m%3:#s-#m%2]if[m=~"nope|yep"];{s+=""s()|s[-1].=_}if[m="oh"];{t=s[-1]y=t..""a=t+0;a=a..""{s[-1]=ord(s)}if[#a>#y]else{s[-1]=chr(t)}}if[#m=6]}_;[s[-1]]}

Try it online!

Explanation

#variable returns the length of variable (if it is a string or an array).

f c,n{                         /*declare a function f with arguments c and n*/
s=[];                          /*initialise the stack*/
(c/`\W|_`)                     /*split the code on anything not [a-zA-Z0-9]*/
          ()|                  /*and push each of its values to the stream*/
{|m|...}_                      /*for each element m in the stream, do:*/
s+=#m-2 if[m=~"yes|no"];       /* add 1 or 0 to the stack if m is "yes" or "no"*/
s[-1]+=#m*2-7if[m=~"sure|nah"];/* increment or decrement the top element if m is "sure" or "nah"*/
s+=n if[m="what"];             /* push input if m is "what"*/
s=s[#m%3:#s-#m%2]              /* remove the first or last element of the stack
  if[m=~"nope|yep"];           /* if m is "nope" or "yep" */
{                 }if[m="oh"]; /* if m is "oh" do:*/
 s+=""                         /*  add an element to the stack*/
      s()|s[-1].=_             /*  for each element in s, concatenate that amount to the last element of the stack*/
{                   }if[#m=6]  /* if m is "really" (it's length is 6) do:*/
 t=s[-1]y=t..""                /*  get the last element of the stack*/
 a=t+0;a=a..""                 /*  add 0 to it, if a is a number, this does nothing, otherwise this makes a longer by 1 character*/
 {s[-1]=ord(s)}if[#a>#y]       /*  if a is longer than t (the last element is a char/string) replace it with its code point*/
 else{s[-1]=chr(t)}            /*  otherwise, replace the last element with the char it represents*/
 [s[-1]]                       /*finally output the top of the stack*/

Answer 3

05AB1E, 77 67 63 61 bytes

Að«Ã#vyÇO§}ðý•9ǝ×н}ÀÀÙ™Íð•650в"X ¾ I > < DdiçëÇ} J r\r \"#:.V

Assumes the program is on the top of the stack.

Try it online!

Answer 4

Pyth, 69 bytes

Vczd=Y.v@c"+YsY X_1Y1 +Yw  +Y0 X_1Y_1    PY   +Y1 tY +PYCeY"d%CN23;eY

Try it online!

Answer 5

JS (ES6), 361 340 bytes

c=>i=>(s=[],r="s[s.length-1]",c.replace(/[^a-z ]/g,"").split` `.map(x=>(eval({"yes":"s.push(1)","no":"s.push(0)","what":"s.push(i)","sure":"~++","nah":"~--","really":"~=((typeof ~)[0]<'o'?String.fromCharCode(~):lastItem.charCodeAt())","oh":"s.push(s.reduce((x,y)=>''+x+y))","nope":"s.shift()","yep":"s.pop()"}[x].replace(/~/g,r)))),eval(r))

Takes code and input through currying.

var yes = 
c=>i=>(s=[],r="s[s.length-1]",c.replace(/[^a-z ]/g,"").split` `.map(x=>(eval({"yes":"s.push(1)","no":"s.push(0)","what":"s.push(i)","sure":"~++","nah":"~--","really":"~=((typeof ~)[0]<'o'?String.fromCharCode(~):lastItem.charCodeAt())","oh":"s.push(s.reduce((x,y)=>''+x+y))","nope":"s.shift()","yep":"s.pop()"}[x].replace(/~/g,r)))),eval(r))

textarea{
  display: block;
}

Code: <textarea id = "code"></textarea>
Input: <textarea id = "input"></textarea>
<button onclick = "output.value = yes(code.value)(input.value)">Run</button>
<textarea id = "output"></textarea>

Answer 6

JavaScript (ES6), 220 216 bytes

c=>i=>c.replace(/\w+/g,x=>a=a.concat(eval("Q,[]  (b=Q)[0]?b.charCodeAt():String.fromCharCode(b) 1    [a.join``,a=[]][0] Q+1 a.shift(),[]  i  Q-1".split`Q`.join`a.pop()`.split` `[parseInt(x,36)%19]||0)),a=[])&&a.pop()

A function that takes input with currying syntax, like f(code)(input).

Answer 7

Python 2, 258 bytes

s=[]
m=-1
i,p=input()
for c in p.split(' '):
 k=c<"o"
 if c in"yesnowhat":s+=[i if"w"==c[0]else-k+1]
 if c in"surenah":s[m]+=[1,m][k]
 if"p"in c:s.pop(k-1)
 if"oh"==c:s+=[''.join(map(str,s))]
 if"y"==c[m]:s[m]="'"in`s[m]`and ord(s[m])or chr(s[m])
print(s[m])

Try it online!

---

-3 bytes thanks to @Wondercricket

Answer 8

Perl 6,  233  225 bytes

{my @s;{yes=>{@s.push(1)},no=>{@s.push(0)},what=>{@s.push: once slurp},sure=>{++@s.tail},nah=>{--@s.tail},really=>{($/:=@s.tail)~~Int??$/.=chr!!$/.=ord},oh=>{@s.=join},nope=>{@s.shift},yep=>{@s.pop},}{.words}.map:{.()};@s.tail.print}

Try it
Hello World
cat
-1
nul
cat_
0
1

{my @s;{es=>{@s.push(1)},no=>{@s.push(0)},at=>{@s.push: once slurp},re=>{++@s.tail},ah=>{--@s.tail},ly=>->{($_:=@s.tail)~~Int??.=chr!!.=ord},oh=>{@s.=join},pe=>{@s.shift},ep=>{@s.pop},}{.comb(/..»/)}.map:{.()};@s.tail.print}

Works the same except it only grabs the last two characters from each command, and uses $_ instead of $/ for really.

Try it (Hello World)

Expanded:

{
  my @s; # stack

  {  # Associative array
    yes    => {@s.push(1)},
    no     => {@s.push(0)},
    what   => {@s.push: once slurp}, # read everything from $*IN
    sure   => {++@s.tail},
    nah    => {--@s.tail},
    really => {
          ( $/ := @s.tail ) # bind $/ to the last value in the stack
          ~~ Int            # if that is an Int
      ??  $/.=chr           # replace it with that character
      !!  $/.=ord           # otherwise replace it with its ordinal
    },
    oh     => {@s.=join},
    nope   => {@s.shift},
    yep    => {@s.pop},
  }\
  { .words }                # index by the words in the program
  .map: {.()};              # call each of the lambdas in order

  @s.tail.print             # print the last value on the stack
}

Answer 9

PHP, 315 305 bytes

second draft, not tested yet

foreach($argv as$k=>$v)if($k>1)eval((strstr($c=preg_replace('#[^a-z ]#','',$v),p)?'':'$s[]=').[yes=>1,no=>0,what=>'$argv[1]',sure=>'array_pop($s)+1',nah:'array_pop($s)-1',really=>'is_int($x=array_pop($s))?chr($x):ord($x)',oh=>'join($s)',nope=>'array_shift($s)',yep=>'array_pop($s)'][$c].';');echo end($s);

Run with php -nr '<php-code>' <input> <yes-code>.

breakdown

foreach($argv as$k=>$v)if($k>1)         # loop through commands
    eval(                                   # 3. interprete
        (strstr(                            # 2. if no 'p' in command, prepend '$s[]='
                                            # 1. ignore all non-code characters
            $c=preg_replace('#[^a-z ]#','',$v),p)?'':'$s[]=').
        [yes=>1,                                # yes: append 1
        no=>0,                                  # no: append 0
        what=>'$argv[1]',                       # what: append input
        sure=>'array_pop($s)+1',                # sure: remove end, increment, append
        nah:'array_pop($s)-1',                  # nah: remove end, decrement, append
                                                # really: convert between ascii and ordinal
        really=>'is_int($x=array_pop($s))?chr($x):ord($x)',
        oh=>'join($s)',                         # oh: concatenate elements, append
        nope=>'array_shift($s)',                # nope: remove first element
        yep=>'array_pop($s)']                   # yep: remove last element
        [$c]
    .';');
echo end($s);                           # print last element (if exists)

Answer 10

PowerShell, 286 bytes

$s=@()
$k-split"\s"|%{
switch($_){
"yes"{$s+=1}
"no"{$s+=0}
"what"{$s+=(read-host)}
"sure"{$s[-1]+=1}
"nah"{$s[-1]-=1}
"really"{if($s[-1]-is[int]){[char]($s[-1])}else{[int][char]($s[-1])}}
"oh"{$s+=($s-join'')}
"nope"{$s=($s[1..($s.Length-1)])}
"yep"{$s=($s[0..($s.Length-2)])}}}
$s[-1]

Try it online!

The input is defined as $k in header of TIO.

An empty array $s initialized.

Splits the input on whitespace using a regex.

Then for each switch it does

yes: Push 1 to the stack (read stack as array and push as append, += operator used, which adds an item at the end of arrays (here))
no: Push 0 to the stack
what: Push the input to the stack
sure: Increment the last item in the stack (indexing and += used)
nah: Decrement the last item in the stack
really: If the last stack item is a number, replace it with its Unicode character. If it is a letter, replace it with its Unicode char code. (used [char] and [int][char] type casting)
oh: convert the stack to strings and concatenate it, and push that to the stack. (Just join the array with '')
nope: remove the first stack item (Using indexing)
yep: remove the last stack item (Using indexing)

At last output last item in array.