# `Expresso.Overlay.PropertyVerifier`
[🔗](https://github.com/rellen/expresso/blob/main/lib/expresso/overlay/property_verifier.ex#L1)

The Spark verifier of the custom properties of the overlays

It runs after the transformers, one time for each deck module at compile
time. It calls `Expresso.Overlay.Properties.slide/2` for each slide with the
names of the theme and of the CSS of the deck, and it gives each result to
the compiler as a warning.

A warning is not an error. The deck compiles, and the renderer writes the
property. Elixir reports the warning of a verifier as a compile warning, and
`mix compile --warnings-as-errors` then fails for a deck module in `lib/`. A
test collects the warning with `Spark.Test.dsl_warnings/1`.

# `verify`

```elixir
@spec verify(map()) :: :ok | {:warn, [Expresso.Overlay.Properties.warning()]}
```

Give a warning for each custom property of the deck that is a defect

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*Consult [api-reference.md](api-reference.md) for complete listing*