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

The Spark transformer for overlays

It runs one time for each deck module at compile time. It calls
`Expresso.Overlay.Expand.slide/1` for each slide, and it writes the result
into the DSL state. An error from the expansion becomes a
`Spark.Error.DslError` with the path of the slide.

# `after?`

# `after_compile?`

# `before?`

# `transform`

```elixir
@spec transform(map()) :: {:ok, map()} | {:error, Spark.Error.DslError.t()}
```

Expand the specifications of each slide of the deck

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