# Functions in data structure

**URL:** <https://forum.canton.network/t/functions-in-data-structure/1417>\
**Category:** App Development\
**Tags:** daml, ledger-api\
**Created:** [October 7, 2020, 1:00pm UTC](https://forum.canton.network/t/functions-in-data-structure/1417 "2020-10-07T13:00:00Z")\
**Posts on this page:** 1\
**Showing post:** 10

<div class="post-metadata">

**Author:** ![Gary\_Verhaegen](https://yyz2.discourse-cdn.com/flex034/user_avatar/forum.canton.network/gary_verhaegen/32/81_2.png) [@Gary\_Verhaegen](https://forum.canton.network/u/Gary_Verhaegen)\
**Post date:** [October 8, 2020, 12:16pm UTC](https://forum.canton.network/t/functions-in-data-structure/1417/10 "2020-10-08T12:16:07Z")

</div>

> [@bernhard](#):
>
> It makes it impossible to dynamically extend functionality without upgrading templates.

`s/impossible/cumbersome/`

```haskell
module Main where

import Daml.Script

data Formula =
    Arg1
  | Arg2
  | Plus with arg1: Formula, arg2: Formula
  | Minus with arg1: Formula, arg2: Formula
  | Times with arg1: Formula, arg2: Formula
  deriving (Eq, Show)

eval: Formula -> Int -> Int -> Int
eval f a b = case f of
  Arg1 -> a
  Arg2 -> b
  Plus f1 f2 -> (eval f1 a b) + (eval f2 a b)
  Minus f1 f2 -> (eval f1 a b) - (eval f2 a b)
  Times f1 f2 -> (eval f1 a b) * (eval f2 a b)

template Calculator
  with
    owner: Party
    formula: Formula
    obs: Party
  where
    signatory owner
    observer obs
    nonconsuming choice Eval: Int
      with a : Int, b: Int
      controller obs
      do
        return $ eval formula a b

setup : Script ()
setup = script do
  alice <- allocatePartyWithHint "Alice" (PartyIdHint "Alice")
  bob <- allocatePartyWithHint "Bob" (PartyIdHint "Bob")

  c1 <- submit bob do createCmd Calculator with owner = bob, obs = alice, formula = (Plus Arg1 Arg2)
  c2 <- submit bob do createCmd Calculator with owner = bob, obs = alice, formula = (Minus (Times Arg1 Arg1) (Times Arg2 Arg2))

  res1 <- submit alice do exerciseCmd c1 (Eval 3 4)
  res2 <- submit alice do exerciseCmd c2 (Eval 3 4)

  assert (res1 == 7)
  assert (res2 == -7)

  return ()

```

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