We will utilize the basic calculator app logic from the parsing guide to produce a linked calculator agent %rpn supporting the following operators by the appropriate parsers:

numbers (as @rs without . dot prefix) (royl-rs:so)
+ lus, addition (lus)
- hep, subtraction (hep)
* tar, multiplication (tar)
/ fas, division (fas)
. dot, display top of stack (dot)

We will leave all regular Gall arms as their defaults, but of course poking, subscribing, and peeking should be supported in a full application.

Agent Logic

/sur/rpn.hoon

We just need to define the expected operators that will show up in the stack. These are @t text constants.

|%
+$  op  $?  [%op %add]
            [%op %sub]
            [%op %mul]
            [%op %div]
            [%op %sho]
        ==
+$  num  @rs
+$  command  ?(@rs op)
--

(+$command doesn't really feel like the right name here, but we're pattern-matching with the demo /app/shoe.hoon.)

/lib/rpn.hoon

These are the parsing rules that the CLI agent will use. We could include these directly in the agent file but we'll post them to a library file.

|%
++  num     royl-rs:so
++  op-add  (cook |=(p=@ ?:(=('+' p) op+%add ~)) lus)
++  op-sub  (cook |=(p=@ ?:(=('-' p) op+%sub ~)) hep)
++  op-mul  (cook |=(p=@ ?:(=('*' p) op+%mul ~)) tar)
++  op-div  (cook |=(p=@ ?:(=('/' p) op+%div ~)) fas)
++  op-sho  (cook |=(p=@ ?:(=('.' p) op+%sho ~)) dot)
++  ops     ;~(pose op-add op-sub op-mul op-div op-sho)
--

/app/rpn.hoon

++  state-0
  $:  %0
      stack=(list ?(@rs op:rpn))
  ==

++command-parser

We want this arm to wait until RETURN is pressed so we ++stag the value with | FALSE/%.n.

++  command-parser
  |=  =sole-id:shoe
  ^+  |~(nail *(like [? command:rpn]))
  %+  stag  |
  (cook command:rpn ;~(pose num:rpnlib ops:rpnlib))

++on-command

This arm pushes values onto the stack, displays the stack, then checks to parse for the result of an operation.

++  on-command
  |=  [=sole-id:shoe =command:rpn]
  ^-  (quip card _this)
  =/  old-stack  (weld stack ~[command])
  =/  new-stack  (process:rpnlib old-stack)
  :_  this(stack new-stack)
  :~  [%shoe ~ sole+klr+~[(crip "{<old-stack>} →")]]
      [%shoe ~ sole+klr+~[[[`%br ~ `%g] (crip "{<new-stack>}") ~]]]
  ==

For this we add a helper arm to /lib/rpn.hoon which takes each entry, makes sure it is a @rs atom, and carries out the operation. (This could probably be made more efficient.)

/lib/rpn.hoon

/-  rpn
:: * * *
++  process
  |=  stack=(list command:rpn)
  ^-  (list command:rpn)
  ~|  "Failure processing operation on stack {<stack>}"
  ?~  stack  !!
  ?-    `command:rpn`(snag 0 (flop stack))
      [%op %add]
    =/  augend        ;;(@rs `command:rpn`(snag 1 (flop stack)))
    =/  addend        ;;(@rs `command:rpn`(snag 2 (flop stack)))
    (flop (weld ~[(add:rs augend addend)] (slag 3 (flop stack))))
    ::
      [%op %sub]
    =/  minuend       ;;(@rs `command:rpn`(snag 1 (flop stack)))
    =/  subtrahend    ;;(@rs `command:rpn`(snag 2 (flop stack)))
    (flop (weld ~[(sub:rs minuend subtrahend)] (slag 3 (flop stack))))
    ::
      [%op %mul]
    =/  multiplicand  ;;(@rs `command:rpn`(snag 1 (flop stack)))
    =/  multiplier    ;;(@rs `command:rpn`(snag 2 (flop stack)))
    (flop (weld ~[(mul:rs multiplicand multiplier)] (slag 3 (flop stack))))
    ::
      [%op %div]
    =/  numerator     ;;(@rs `command:rpn`(snag 1 (flop stack)))
    =/  denominator   ;;(@rs `command:rpn`(snag 2 (flop stack)))
    (flop (weld ~[(div:rs numerator denominator)] (slag 3 (flop stack))))
    ::
      [%op %sho]
    ~&  >  "{<(snag 1 (flop stack))>}"
    (flop (slag 1 (flop stack)))
    ::
      @rs
    stack
  ==

Linking

After a %sole agent has been |installed, it should be registered for Dojo to cycle input to it using |link.

|link %rpn

Now Ctrl+X allows you to switch to that app and evaluate expressions using it.

gall: booted %rpn
> 50
~ →
~[.50]

> 25
~[.50] →
~[.50 .25]

> -
~[.50 .25] →
~[.-25]

> 5
~[.-25] →
~[.-25 .5]

> /
~[.-25 .5] →
~[.-0.19999999]

> 5
~[.-0.19999999] →
~[.-0.19999999 .5]

> *
~[.-0.19999999 .5] →
~[.-0.99999994]

> 1
~[.-0.99999994] →
~[.-0.99999994 .1]

> /
~[.-0.99999994 .1] →
~[.-1]

Exercises

Extend the calculator app to support modulus as % cen.
Extend the calculator app so it instead operates on @rd values. Either use ++cook to automatically convert the input values from a 1.23-style input to the .~1.23 @rd style or build a different input parser from the entries in ++royl:so.
Extend the calculator app so that it can support named variables (using @tas) with = tis. What new data structure do you need? For convenience, expose the result of the last operation as ans (a feature of TI graphing calculators and MATLAB, among other programs).
The calculator app stack isn't really a proper CS stack with push and pop operations. Refactor it to use such a type.

Building a CLI App

Agent Logic

Linking

Exercises