Input/output contract for an Imp program.

A signature names the fields a program receives and the fields it must produce. It is the center of the Imp programming model: adapters render it for models, schemas validate structured outputs, optimizers mutate programs around it, and persistence stores it as plain data.

Required input presence is enforced before an LM call. To preserve DSPy's calling semantics, supplied input values that disagree with an explicitly declared type warn by default and still proceed. Output values are parsed and validated strictly. Validate untrusted application inputs at your own boundary when rejection is required.

Field names written in the string syntax are atoms only if that atom already exists, otherwise strings, so read values with Imp.get/2.

The compact string form is ideal for most code:

iex> signature = Imp.Signature.new("question: string -> answer: short_span")
iex> Imp.Signature.input_names(signature)
[:question]
iex> Imp.Signature.output_names(signature)
[:answer]
iex> Imp.Signature.json_schema(signature)["properties"]["answer"]["x-imp-answerShape"]
:short_span

Use the map form when constraints or metadata should be explicit data:

iex> signature =
...>   Imp.Signature.new(%{
...>     inputs: [:text],
...>     outputs: [
...>       %{name: :sentiment, type: :string, constraints: %{enum: ["positive", "negative"]}}
...>     ]
...>   })
iex> Imp.Signature.json_schema(signature)["properties"]["sentiment"]["enum"]
["positive", "negative"]