Don't duplicate work.md

imports

module Plutarch.Docs.WorkDuplication (abs, abs', pf, pf') where 
import Plutarch.Prelude
import Prelude hiding (abs)

Don't duplicate work

Haskell bindings are simply "inlined" during Plutarch compilation.

Consider the simple snippet:

pf :: Term s PInteger
pf =
  let 
    foo :: forall s. Term s PInteger
    foo = 1 + 2      -- | A Haskell binding.
  in pif
       (foo #== 3)     -- | A.) ...then inline here...
       foo             -- | B.) ...and inline here.
       7

Using the printTerm function (provided by the top level Plutarch module), we can view the computation bound to foo. The formatting below is our own; notice that foo, which becomes (addInteger 1 2) in UPLC, is inlined twice:

> printTerm pf

(...)

(force
    (force ifThenElse
       (equalsInteger
          (addInteger 1 2)      -- | A.) `foo` appears here...
          3
       )
       (delay (addInteger 1 2)) -- | B.) ...and here
       (delay 7)
    )
)

Performing this computation twice is obviously bad (in this circumstance), since it will increase the execution budget for the script.

A technique to circumvent this is to introduce a free variable via a lambda, replace the inlined expression (in our case, (addInteger 1 2)) with that variable, and them apply the lambda to the calculated expression:

> printTerm pf'

(...)

((\\i0 ->                          -- | A'.) Introduce a lambda here,...
    force
        (force ifThenElse
            (equalsInteger i1 3)   -- | B'.) ...apply the argument here,...
            (delay i1)             -- | C'.) ...and apply the argument here,
            (delay 7)
        )
 ) (addInteger 1 2)                -- | D'.) ...then calculate `foo` once and apply the lambda
)

Plutarch provides the plet :: Term s a -> (Term s a -> Term s b) -> Term s b function to accomplish exactly this. To demonstrate this technique, the implementation of pf' that will lead to the above UPLC is given as:

{-
Note: the letter labels on our annotations match the operations in the
previous example.
-}

pf' :: Term s PInteger
pf' =
  plet (1 + 2) $             -- | D.') Calculate the desired value here (strictly),...
  \foo ->                    -- | A.') ...introduce a lambda abstraction,...
    pif
      (foo #== 3)            -- | B.') ...and apply the argument here...
      foo                    -- | C.') ... and here.
      7

Another example of this would be:

abs :: Term s PInteger -> Term s PInteger
abs x = pif (x #<= -1) (negate x) x

Guess what would happen if you used it like:

abs (reallyExpensiveFunction # arg)

It'd turn into:

pif ((reallyExpensiveFunction # arg) #<= -1) (negate (reallyExpensiveFunction # arg)) (reallyExpensiveFunction # arg)

Oh no. reallyExpensiveFunction is going to be applied three times. That's 3 times the cost!

Instead, consider using plet:

abs' :: Term s PInteger -> Term s PInteger
abs' x' = plet x' $ \x -> pif (x #<= -1) (negate x) x

Of course, what you really should do , is prefer Plutarch level functions whenever possible. Since arguments to Plutarch level functions are pre-evaluated and those bindings are completely ok to use as many times as you want!

Where should arguments be pleted?

You don't have to worry about work duplication on arguments in every single scenario. In particular, the argument to plam is also a Haskell function, isn't it? But you don't need to worry about pleting your arguments there since it becomes a Plutarch level function through plam - thus, all the arguments are evaluated before being passed in.

Where else is plet unnecessary? Functions taking in continuations, such as plet (duh) and pletFields, always pre-evaluate the binding. An exception, however, is pmatch. In certain cases, you don't need to plet bindings within the pmatch case handler. For example, if you use pmatch on a PList, the x and xs in the PSCons x xs will always be pre-evaluated. On the other hand, if you use pmatch on a PBuiltinList, the x and xs in the PCons x xs are not pre-evaluated. Be sure to plet them if you use them several times!

In general, pleting something back-to-back several times will be optimized to a singular plet anyway. However, you should know that for data encoded types (types that follow "implementing PIsDataRepr and friends") and Scott encoded types, pmatch handlers get pre-evaluated bindings. For PBuiltinList, and PDataRecord - the bindings are not pre-evaluated.

You should also plet local bindings! In particular, if you applied a function (Plutarch level or Haskell level) to obtain a value, then bound that value to a variable e.g. with let or where, then avoid using it multiple times. The binding will simply get inlined as the function application - and it'll keep getting re-evaluated. You should plet it first!

This also applies to field accesses using OverloadedRecordDot. When you do ctx.purpose, it really gets translated to getField @"purpose" ctx, which is a function call! If you use the field multiple times, plet it first.

Another slightly obscure case can be observed in scott encoded types. When you build a scott encoded type using pcon - the Plutarch terms you use as fields are simply inlined within the scott encoded type. As such, pcon $ PPair <complex expr> <another complex expr> ends up like:

(\f -> f <complex expr> <another complex expr>)

This is practically pseudocode. However, it demonstrates that your expressions are not evaluated when building the scott encoded pair. Indeed, they will be evaluated when you pmatch on it. As such, if you pmatch on this pair multiple times, those expressions will evaluate multiple times!

If you must pmatch on such types several times, plet the fields before building the container type!