+

+ is HQ9+ with two differences: there are no "H", "Q", or "9" instructions, and all characters that are not + are explicitly ignored.

Instructions

Unlike some languages, + is relatively simple to learn, even for non-programmers. It has only one instruction, +, which increments the program's accumulator. All others are ignored.

Examples

Increment the accumulator once:

+

Increment it five times:

+++++

Wow, what a useful language!

Computational class

+ is a push-down automaton, as the + instruction simulates pushing to a stack. The initial state can also be a halt state, and the only state, and a + instruction can loop back to the initial/halt/only state, pushing a value to the stack as it does so.

Interpreters

Perl

60 bytes

my$r=$ARGV[0];open my$z,'<',$r or die$!;my$c=()=<$z>=~/\+/g;

XENBLN

17 bytes, 12 characters

$a0ŽIÍ=x„+äa

Lua

Reads program from given file name.

local args = {...}
local file = io.open(args[1],"r")
local counter = 0
while true do
	local nLine = file:lines()()
	if not nLine then
		break
	end
	if string.find(nLine,"+") then
		local _, count = string.gsub(nLine,"+","+")
		counter = counter + count
	end
end
file:close()

Shorter, less… stupid. Reads program from input.

local counter = 0
repeat
	local c = io.read(1)
	if c == '+' then counter = counter + 1 end
until not c

Even shorter. Requires input to terminate before execution starts. Entire program is stored in memory, so beware long programs.

local counter = #io.read("*a"):gsub("[^%+]","")

><>

Reads code from input

>i:01-=?v00.
v  &0r+1<
>:"+"=?!v&1v
^  ~;!?:<&+<

A second version

0&01.
>i:0(?;"+"=?v
^       &+1&<

Python 3

a = 0
c = input()
for char in c:
    a += 1 if char == "+" else 0

The one-liner:

(a:=0,[a:=a+(_=='+')for _ in input()])

Shorter:

input().count("+")