`spend` fails without descriptive error

[Renegatto]

Plutus.Model.Contract.spend fails with:

Exception: Maybe.fromJust: Nothing
CallStack (from HasCallStack):
  error, called at libraries/base/Data/Maybe.hs:149:21 in base:Data.Maybe
  fromJust, called at src/Plutus/Model/Contract.hs:276:10 in plutus-simple-model-0.5.2.0-4fEbkvuS1hh58koi6xqmzV:Plutus.Model.Contract

It forces user to dive into PSM source code to figure out the reason of the error, which is not the best user experience.

Proposed solution

Make a descriptive error message.
Maybe even take optional error message as a function parameter.

[AgustinBadi]

Hi @Renegatto !

There's a function called withSpend :: PubKeyHash -> Value -> (UserSpend -> Run ()) -> Run () that serves the same purpose as spend and logs an error if an user doesn't have enough funds. Maybe that can be an awnser to your requirement, or could be that there are other errors that should be catched from your point of view?

Greetings,

[Renegatto]

Hi, @AgustinBadi.
It's good that such a function exists, but in general in a program (especially in a library) when a function fails it should be clear where and/or why.
I think Exception: Maybe.fromJust: Nothing is not something we expect to see when using library which is not base.

The solution is simply throwing custom (not necessary defined by the library user) error on Nothing instead of using fromJust.