19.24 Dynamic type cast expression _dynamic $\mathrm{⟨}$exp$\mathrm{⟩}$ as $\mathrm{⟨}$ty$\mathrm{⟩}$

This expression performs dynamic type cast of dynamically-typed value $⟨$exp$\mathrm{⟩}$ to $⟨$ty$\mathrm{⟩}$. The type of $⟨$exp$\mathrm{⟩}$ must be $\tau$ Dynamic.dyn for some $\tau$. The type of this expression is $⟨$ty$\mathrm{⟩}$.

To evaluate this expression, $⟨$exp$\mathrm{⟩}$ is evaluated and the dynamically-typed value $v$ is obtained. The value of this expression depends on the structure of $v$ and $⟨$ty$\mathrm{⟩}$. The rule of dynamic type cast is the following:

• •

  When $⟨$ty$\mathrm{⟩}$ is Dynamic.void Dynamic.dyn, the value of this expression is $v$.

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  When $⟨$ty$\mathrm{⟩}$ is $\tau$ Dynamic.dyn for some $tau$, if $v$ has a view of $\tau$ (a substructure of $v$ can be extracted as a value of $\tau$), the value of this expression is $v$. Otherwise, the Dynamic.RuntimeTypeError excepstion is raised.

• •

  When $⟨$ty$\mathrm{⟩}$ does not include Dynamic.dyn, the type of $v$ is identical to $⟨$ty$\mathrm{⟩}$, the value of this expression is the value obtained by type-casting $v$ to $⟨$ty$\mathrm{⟩}$. Otherwise, the Dynamic.RuntimeTypeError excepstion is raised.

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  When $⟨$ty$\mathrm{⟩}$ includes Dynamic.dyn as its substructure, the above rules are applied recursively on the structure of $v$ and $⟨$ty$\mathrm{⟩}$.

For example, suppose the following list of records:

val r = Dynamic.dynamic [{name = "Joe", age = 21}, {name = "Sue", age = 31}];

The following casts are correct:

_dynamic r as {name:string, age:int} list;
_dynamic r as {name:Dynamic.void Dynamic.dyn, age:int} list;
_dynamic r as Dynamic.void Dynamic.dyn;
_dynamic r as Dynamic.void Dynamic.dyn list;
_dynamic r as {name:string, age:int} Dynamic.dyn list;
_dynamic r as {name:string} Dynamic.dyn list;
_dynamic r as {name:string, age:int} list Dynamic.dyn;
_dynamic r as {age:int} list Dynamic.dyn;

Note that $v$ is not always a data structure that is typable in ML. For example, suppose the following heterogeneous list:

val l = Dynamic.fromJson
          "[{\"name\":\"Joe\", \"age\":21},\
          \{\"name\":\"Sue\", \"grade\":2.0},\
          \{\"name\":\"Robert\", \"nickname\":\"Bob\"}]";

The following casts are correct:

_dynamic l as Dynamic.void Dynamic.dyn;
_dynamic l as Dynamic.void Dynamic.dyn list;
_dynamic l as {name:string} Dynamic.dyn list;
_dynamic l as {name:string} list Dynamic.dyn;
_dynamic l as {name:Dynamic.void Dynamic.dyn} Dynamic.dyn list;

To use this expression in the separate compilation mode, "reify.smi" must be _required.