How to get the funds available at a script using Plutus?

Question

I am currently working on Lecture 8 in the Plutus Pioneer program.

I want to view funds available at a script address.

There appears to be a function that makes this easy in Ledger.AddressMap

-- | Get the funds available at a particular address.
fundsAt :: Address -> Lens' AddressMap UtxoMap
fundsAt addr = at addr . non mempty

However, I have not been able to get this to work.

When I try this

fundsAt scriptAddress

I get the following error

Ambiguous type variable ‘f0’ arising from a use of ‘fundsAt’
  prevents the constraint ‘(Prelude.Functor f0)’ from being solved.

My attempts at trying to make the type unambiguous have not worked, and I am struggling to understand Lens'.

Does anyone know how to get the fundsAt function to work?

Mitchell Turner · Accepted Answer · 2021-08-13 21:31:52Z

2

If you are trying to do this in your offchain code, I would suggest using Plutus.Contract.utxoAt instead.

Here is an example of it being used in the gift code.

answered Aug 13, 2021 at 21:31

Mitchell Turner

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I did see that example, but is there a reason why that's a better approach? It seems like fundsAt is a clearer and more direct way to get the data I want and I'm just struggling with the Lens' type.
– Frank DelPidio
Commented Aug 13, 2021 at 23:50
They both give you a UtxoMap. Notice that the return type of the grab function is the Contract monad, just like utxoAt. It's what's meant to be used in the off-chain transaction-building code. I doubt there is a conversion between Lens' and Contract.
– Mitchell Turner
Commented Aug 14, 2021 at 0:07

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