What is the method for converting the return type of an S-Expression parser into a data structure in SML?

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The method for converting the return type of an S-Expression parser into a data structure in SML can involve pattern matching and recursion.

Assuming the S-Expression parser returns an abstract syntax tree (AST) with different types of nodes, we can define a function that takes the AST node and returns the corresponding data structure.

For example, if the AST node represents a list, we can recursively traverse the list and convert each element into its corresponding data structure. If the AST node represents an integer or a string, we can return the integer or string value.

Here is an example function that converts an AST node into a data structure in SML:

datatype SExpr =
    Int of int
  | Str of string
  | List of SExpr list

fun convert (ast: SExpr): data_structure =
  case ast of
    Int n => n
  | Str s => s
  | List lst => List.map convert lst

In this example, the convert function takes an AST node of type SExpr and returns the corresponding data structure. If the AST node is an integer, it returns the integer value. If it is a string, it returns the string value. If it is a list, it recursively converts each element in the list using the List.map function and returns a list of converted elements.