files/arthur-docs-markdown/user-guide/api-query-guide/composing_functions.md.txt

# Composing Functions (Advanced Usage)

To see functions you can use with this syntax, 
checkout the {doc}`aggregation functions <aggregation_functions>` 
and the {doc}`transformation functions <transformation_functions>`.

Any function with a parameter with the type signature `[string or nested]` 
is able to accept the following as a value:
1. a string constant that represents a property of the model
1. an object of the form: 
    ```json
    {
        "alias_ref": "string",
        "nested_function": {"...":  "..."}
    }
    ```

    Only one of `alias_ref` or `nested_function` may be present at a time. See the following
    explanations of each.

## Alias References
The `alias_ref` field allows specifying another selected column's alias
to use as input to the function. This example uses an `alias_ref` to pull in another column
to the `add` function:
```json
{
    "select": [
        {
            "function": "abs",
            "alias": "absLoan",
            "parameters": {
                "property": "loan"
            }
        },
        {
            "function": "add",
            "alias": "plus2",
            "parameters": {
                "left": 2,
                "right": {
                    "alias_ref": "absLoan"
                }
            }
        }
    ]
}
```

This request returns:
```json
{
    "query_result": [
        {
            "absLoan": 55.45,
            "plus2": 57.45
        }
    ]
}
```

## Nested Functions
The `nested_function` field allows specifying another function definition
to use as input. Here's an example of how to calculate absolute error for
a regression model. In this example, we pass the nested `subtract` function as 
input to the `abs` function via the `nested_function` object for the `property` 
parameter of `abs`:
```json
{
    "select": [
        {
            "function": "abs",
            "alias": "abs_error",
            "parameters": {
                "property": {
                    "nested_function": {
                        "function": "subtract",
                        "alias": "error",
                        "parameters": {
                            "left": "Predicted_FICO_Score",
                            "right": {
                                "alias_ref": "ground_truth"
                            }
                        }
                    }
                }
            }
        },
        {
            "property": "Predicted_FICO_Score"
        },
        {
            "property": "Consumer_Credit_Score",
            "alias": "ground_truth"
        }
    ]
}
```

This query returns:
```json
{
    "query_result": [
        {
            "Consumer_Credit_Score": 660,
            "Predicted_FICO_Score": 688.10004,
            "abs_error": 28.100040000000035
        },
        {
            "Consumer_Credit_Score": 663,
            "Predicted_FICO_Score": 681,
            "abs_error": 18
        },
        "..."
    ]
}
```

```{note} If you use the same function multiple times in a query, you need to give each one a distinct "alias". Otherwise, the names will conflict.
```