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Casts​
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This section describes introspection of Gel type casts. Features like whether the casts are implicit can be discovered by introspecting schema::Cast.

Introspection of the schema::Cast:

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db> 
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... 
... 
... 
... 
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... 
with module schema
select ObjectType {
    name,
    links: {
        name,
    },
    properties: {
        name,
    }
}
filter .name = 'schema::Cast';
{
    Object {
        name: 'schema::Cast',
        links: {
            Object { name: '__type__' },
            Object { name: 'from_type' },
            Object { name: 'to_type' }
        },
        properties: {
            Object { name: 'allow_assignment' },
            Object { name: 'allow_implicit' },
            Object { name: 'id' },
            Object { name: 'name' }
        }
    }
}

Introspection of the possible casts from std::int64 to other types:

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db> 
... 
... 
... 
... 
... 
... 
... 
with module schema
select Cast {
    allow_assignment,
    allow_implicit,
    to_type: { name },
}
filter .from_type.name = 'std::int64'
order by .to_type.name;
{
    Object {
        allow_assignment: false,
        allow_implicit: true,
        to_type: Object { name: 'std::bigint' }
    },
    Object {
        allow_assignment: false,
        allow_implicit: true,
        to_type: Object { name: 'std::decimal' }
    },
    Object {
        allow_assignment: true,
        allow_implicit: false,
        to_type: Object { name: 'std::float32' }
    },
    Object {
        allow_assignment: false,
        allow_implicit: true,
        to_type: Object { name: 'std::float64' }
    },
    Object {
        allow_assignment: true,
        allow_implicit: false,
        to_type: Object { name: 'std::int16' }
    },
    Object {
        allow_assignment: true,
        allow_implicit: false,
        to_type: Object { name: 'std::int32' }
    },
    Object {
        allow_assignment: false,
        allow_implicit: false,
        to_type: Object { name: 'std::json' }
    },
    Object {
        allow_assignment: false,
        allow_implicit: false,
        to_type: Object { name: 'std::str' }
    }
}

The allow_implicit property tells whether this is an implicit cast in all contexts (such as when determining the type of a set of mixed literals or resolving the argument types of functions or operators if there's no exact match). For example, a literal 1 is an int64 and it is implicitly cast into a bigint or float64 if it is added to a set containing either one of those types:

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db> 
select {1, 2n};
{1n, 2n}
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db> 
select {1, 2.0};
{1.0, 2.0}

What happens if there's no implicit cast between a couple of scalars in this type of example? Gel checks whether there's a scalar type such that all of the set elements can be implicitly cast into that:

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db> 
select introspect (typeof {<int64>1, <float32>2}).name;
{'std::float64'}

The scalar types int64 and float32 cannot be implicitly cast into each other, but they both can be implicitly cast into float64.

The allow_assignment property tells whether this is an implicit cast during assignment if a more general implicit cast is not allowed. For example, consider the following type:

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type Example {
    property p_int16: int16;
    property p_float32: float32;
    property p_json: json;
}
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db> 
... 
... 
... 
insert Example {
    p_int16 := 1,
    p_float32 := 2
};
{Object { id: <uuid>'...' }}
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db> 
... 
... 
insert Example {
    p_json := 3  # assignment cast to json not allowed
};
InvalidPropertyTargetError: invalid target for property
'p_json' of object type 'default::Example': 'std::int64'
(expecting 'std::json')