Reimplement: https://github.com/soveran/clac

When a command requires an argument, it pops a value from the stack. If the stack is empty, a zero is provided instead. In the descriptions below, the top of the stack (and thus the first value popped) is represented by the letter a, while the second value popped is represented by the letter b. For example, if the stack is composed of the number 1, 2 and 3 (with 3 at the top of the stack), when we describe the sum then a will be 3 and b will be 2. It is important to note that subtraction and division invert the order of the arguments before performing the operation: with 1, 2 and 3 in the stack, when you type - it will pop the values 3 and 2 and push the result of 2 - 3. This is in the tradition of other postfix calculators and programming languages.

Here's a description of the available commands:

+ Pop two values a and b and push the result of a + b.

- Pop two values a and b and push the result of b - a.

* Pop two values a and b and push the result of a * b.

/ Pop two values a and b and push the result of b / a.

% Pop two values a and b and push the remainder of the Euclidean division of b by a.

^ Pop two values a and b and push the result of b ^ a.

ln Pop the value a and push its natural logarithm.

log Pop the value a and push its logarithm to base 10.

! Pop the value a and push its factorial.

sin Pop the value a and push its sine.

cos Pop the value a and push its cosine.

tan Pop the value a and push its tangent.

asin Pop the value a and push its arc sine.

acos Pop the value a and push its arc cosine.

atan Pop the value a and push its arc tangent.

atan2 Pop two values a and b and push the arc tangent of b / a, using the signs of a and b to determine the quadrant.

erf Pop the value a and push its error function.

sum Pop all the values in the stack and push their sum.

add Pop the value a and remove that many items from the stack. Push their sum.

ceil Pop the value a and push the smallest integer value greater than or equal to a.

floor Pop the value a and push the largest integer value less than or equal to a.

round Pop the value a and push the integer value closest to a.

abs Pop the value a and push the non-negative value of a.

swap Pop two values a and b and push the values a, b.

dup Pop the value a and push the values a, a.

roll Pop two values a and b and rotate b elements in the stack a times.

drop Remove the top of the stack.

clear Remove all the elements in the stack.

count Push the number of items in the stack.

_ Push on the stack the result of the last operation.

stash Pop the value a and move that many items to the stash.

fetch Pop the value a and move that many items from the stash.

. Stash the top of the stack.

, Fetch one stashed item.

: Stash all the items in the stack.

; Fetch all stashed items.

Use <C-p>/<C-n> or the <Up>/<Down> arrow keys to navigate the history.

It is possible to define operations (or words, as they are usually called in stack based programming languages) by editing the words configuration file. It is not created by default, but clac will use some environment variables in order to search for word definitions.

$CLAC_WORDS If set, it should point to a file containing word definitions.

$XDG_CONFIG_HOME If set, clac will search for $XDG_CONFIG_HOME/clac/words.

$HOME If set, clac will search for $HOME/.config/clac/words.

Words are defined as aliases, with one alias on each line. Empty lines are ignored. Here are some examples:

pi 3.14159265358979323846
tau "pi 2 *"
sqrt "0.5 ^"

Note that an alias has two parts: a word to be defined and its meaning. That's why the tau and sqrt definitions are enclosed in double quotes. If the double quotes are removed, clac will complain that it can't parse the command. For example, if we remove the double quotes from sqrt and start clac, we will get this error message:

Incorrect definition: sqrt 0.5 ^
(~/.config/clac/words:3)

User defined words can be used as if they were built-in commands:

$ clac "42 dup * pi *"
5541.76944093239

If you type words and hit enter, clac will list the defined words.

If you type reload and hit enter, clac will reload the words file.

While the most interesting aspect of clac is the ability to visualize the stack as it is updated with each key press, at some point you may want use clac just to get a quick result or call it from a script. For that reason, clac can be used in non-interactive mode by invoking it with an argument.

Here are some examples of non-interactive invocations:

$ clac "3 4 +"
7

$ clac "2 3 4 +"
7
2

When clac finishes evaluating the expression "2 3 4 +", there are two elements in the stack: the number 7 at the top of the stack and the number 2 at the bottom of the stack. The elements are printed in order, one per line, starting from the top of the stack.

This other example uses the stashing features. Let's say we want to push two numbers and get the result of their multiplication plus the square of the second number.

$ clac "4 3 dup dup * . * , +"
21

Another example that uses the stash would be to get the average of all the elements in the stack:

$ clac "1 2 3 4 count . sum , /"
2.5

In fact, if you find yourself calculating averages very often, you can define the word avg as "count . sum , /".