I decided to change from :float to :decimal types in an Phoenix App. And there is a weird case where I post the below mutation:

@desc "Create a wallet"
    field :create_wallet, :wallet do
      arg(:currency, non_null(:currency_type))
      arg(:user_id, non_null(:id))
      arg(:units, :decimal)

      resolve(&Resolvers.WalletResolver.create_wallet/3)
    end

without putting a value for units argument. This raises the error pointing to the Decimal library:

POST /api/graphiql
[debug] Processing with Absinthe.Plug.GraphiQL
  Parameters: %{"query" => "mutation {\n  createWallet(currency: CAD, userId: 1) {\n    id\n    currency\n    units\n    user {\n      id\n      email\n      name\n    }\n  }\n}", "variables" => nil}
  Pipelines: [:api]
[debug] ABSINTHE schema=PaymentServerWeb.Schema variables=%{}
---
mutation {
  createWallet(currency: CAD, userId: 1) {
    id
    currency
    units
    user {
      id
      email
      name
    }
  }
}
---
[debug] QUERY OK source="wallets" db=1.6ms queue=1.1ms idle=1784.6ms
INSERT INTO "wallets" ("currency","user_id","units","inserted_at","updated_at") VALUES ($1,$2,$3,$4,$5) RETURNING "id" [:CAD, 1, 0.0, ~N[2024-04-10 13:43:10], ~N[2024-04-10 13:43:10]]
↳ PaymentServerWeb.Graphql.Resolvers.WalletResolver.create_wallet/3, at: lib/payment_server_web/resolvers/wallet_resolver.ex:23
[debug] QUERY OK source="users" db=1.4ms idle=1790.7ms
SELECT u0."id", u0."email", u0."name", u0."inserted_at", u0."updated_at", u0."id" FROM "users" AS u0 WHERE (u0."id" = $1) [1]
↳ Dataloader.Source.Dataloader.Ecto.run_batch/2, at: lib/dataloader/ecto.ex:703
[info] Sent 500 in 86ms
[error] #PID<0.494.0> running Phoenix.Endpoint.SyncCodeReloadPlug (connection #PID<0.478.0>, stream id 2) terminated
Server: localhost:4000 (http)
Request: POST /api/graphiql
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in Decimal.to_string/2
        (decimal 2.1.1) lib/decimal.ex:1362: Decimal.to_string(0.0, :scientific)
        (absinthe 1.7.6) lib/absinthe/phase/document/result.ex:55: Absinthe.Phase.Document.Result.data/2
        (absinthe 1.7.6) lib/absinthe/phase/document/result.ex:104: Absinthe.Phase.Document.Result.field_data/3
        (absinthe 1.7.6) lib/absinthe/phase/document/result.ex:22: Absinthe.Phase.Document.Result.process/2
        (absinthe 1.7.6) lib/absinthe/phase/document/result.ex:11: Absinthe.Phase.Document.Result.run/2
...

Moreover, the wallet was created successfully with the default units value set to 0.0.

If I pass is the value for unitsin the GraphQL mutation above, it works without error:

debug] Processing with Absinthe.Plug.GraphiQL
  Parameters: %{"query" => "mutation {\n  createWallet(currency: GBP, userId: 1, units: 30.21) {\n    id\n    currency\n    units\n    user {\n      id\n      email\n      name\n    }\n  }\n}", "variables" => nil}
  Pipelines: [:api]
[debug] ABSINTHE schema=PaymentServerWeb.Schema variables=%{}
---
mutation {
  createWallet(currency: GBP, userId: 1, units: 30.21) {
    id
    currency
    units
    user {
      id
      email
      name
    }
  }
}
---
[debug] QUERY OK source="wallets" db=2.7ms idle=1577.6ms
INSERT INTO "wallets" ("currency","user_id","units","inserted_at","updated_at") VALUES ($1,$2,$3,$4,$5) RETURNING "id" [:GBP, 1, Decimal.new("30.21"), ~N[2024-04-10 13:51:15], ~N[2024-04-10 13:51:15]]
↳ PaymentServerWeb.Graphql.Resolvers.WalletResolver.create_wallet/3, at: lib/payment_server_web/resolvers/wallet_resolver.ex:23
[debug] QUERY OK source="users" db=1.3ms idle=1583.3ms
SELECT u0."id", u0."email", u0."name", u0."inserted_at", u0."updated_at", u0."id" FROM "users" AS u0 WHERE (u0."id" = $1) [1]

A simple test proves it as well:

...
@create_wallet_mutation """
  mutation ($currency: CurrencyType!, $userId: ID!, $units: Decimal) {
    createWallet(currency: $currency, userId: $userId, units: $units) {
      currency
      units
      user {
        id
        email
      }
    }
  }
  """
...
test "it should create a new Wallet with valid attributes", %{conn: _conn} do
    user = insert(:user)

    wallet_input = %{
      "currency" => "GBP",
      "userId" => user.id,
      "units" => 0 # remove this argument and the test will fail!
    }

    {:ok, ref} =
      Absinthe.run(@create_wallet_mutation, PaymentServerWeb.Schema, variables: wallet_input)

    expected = %{
      data: %{
        "createWallet" => get_in(ref, [:data, "createWallet"])
      }
    }

    assert ref == expected
  end

Any ideas?