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fix: update the doc to clarify that function names are case-sensitive (…
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scgkiran authored Jan 6, 2025
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A function is a scalar function if that function takes in values from a single record and produces an output value. To clearly specify the definition of functions, Substrait declares an extensible specification plus binding approach to function resolution. A scalar function signature includes the following properties:

| Property | Description | Required |
| ---------------------- | ------------------------------------------------------------ | ----------------------------------- |
| Name | One or more user-friendly UTF-8 strings that are used to reference this function. | At least one value is required. |
| List of arguments | Argument properties are defined below. Arguments can be fully defined or calculated with a type expression. See further details below. | Optional, defaults to niladic. |
| Deterministic | Whether this function is expected to reproduce the same output when it is invoked multiple times with the same input. This informs a plan consumer on whether it can constant-reduce the defined function. An example would be a random() function, which is typically expected to be evaluated repeatedly despite having the same set of inputs. | Optional, defaults to true. |
| Session Dependent | Whether this function is influenced by the session context it is invoked within. For example, a function may be influenced by a user who is invoking the function, the time zone of a session, or some other non-obvious parameter. This can inform caching systems on whether a particular function is cacheable. | Optional, defaults to false. |
| Variadic Behavior | Whether the last argument of the function is variadic or a single argument. If variadic, the argument can optionally have a lower bound (minimum number of instances) and an upper bound (maximum number of instances). | Optional, defaults to single value. |
| Nullability Handling | Describes how nullability of input arguments maps to nullability of output arguments. Three options are: `MIRROR`, `DECLARED_OUTPUT` and `DISCRETE`. More details about nullability handling are listed below. | Optional, defaults to `MIRROR` |
| Description | Additional description of function for implementers or users. Should be written human-readable to allow exposure to end users. Presented as a map with language => description mappings. E.g. `{ "en": "This adds two numbers together.", "fr": "cela ajoute deux nombres"}`. | Optional |
| Return Value | The output type of the expression. Return types can be expressed as a fully-defined type or a type expression. See below for more on type expressions. | Required |
| Property | Description | Required |
| ---------------------- |-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| ----------------------------------- |
| Name | One or more user-friendly, case-sensitive UTF-8 strings that are used to reference this function. | At least one value is required. |
| List of arguments | Argument properties are defined below. Arguments can be fully defined or calculated with a type expression. See further details below. | Optional, defaults to niladic. |
| Deterministic | Whether this function is expected to reproduce the same output when it is invoked multiple times with the same input. This informs a plan consumer on whether it can constant-reduce the defined function. An example would be a random() function, which is typically expected to be evaluated repeatedly despite having the same set of inputs. | Optional, defaults to true. |
| Session Dependent | Whether this function is influenced by the session context it is invoked within. For example, a function may be influenced by a user who is invoking the function, the time zone of a session, or some other non-obvious parameter. This can inform caching systems on whether a particular function is cacheable. | Optional, defaults to false. |
| Variadic Behavior | Whether the last argument of the function is variadic or a single argument. If variadic, the argument can optionally have a lower bound (minimum number of instances) and an upper bound (maximum number of instances). | Optional, defaults to single value. |
| Nullability Handling | Describes how nullability of input arguments maps to nullability of output arguments. Three options are: `MIRROR`, `DECLARED_OUTPUT` and `DISCRETE`. More details about nullability handling are listed below. | Optional, defaults to `MIRROR` |
| Description | Additional description of function for implementers or users. Should be written human-readable to allow exposure to end users. Presented as a map with language => description mappings. E.g. `{ "en": "This adds two numbers together.", "fr": "cela ajoute deux nombres"}`. | Optional |
| Return Value | The output type of the expression. Return types can be expressed as a fully-defined type or a type expression. See below for more on type expressions. | Required |
| Implementation Map | A map of implementation locations for one or more implementations of the given function. Each key is a function implementation type. Implementation types include examples such as: AthenaArrowLambda, TrinoV361Jar, ArrowCppKernelEnum, GandivaEnum, LinkedIn Transport Jar, etc. [Definition TBD]. Implementation type has one or more properties associated with retrieval of that implementation. | Optional |


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