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The Sym Programming Language
The Sym programming language is designed first and foremost for manipulating expressions. Because it's designed to manipulate expressions, the code in the Sym language is built from Sym expressions rather than text.
Pattern Matching - Overview
The purpose of a pattern match is to bind an expression to a variable during the execution of code. Most commonly, the expressions that become the values bound variables are the parameters passed into a function call.
Objects Used in Pattern Matching
There are a number of objects used in the pattern matching process, depending on what type of match is required.
The MatchIn Object
The MatchIn object is designed to allow creating pattern match expressions that will generate a list of bindings for each of the parameters of an expression. Among other uses, the MatchIn object provides a way to iterate over a list.

The Sym Programming Language
The Sym programming language is designed first and foremost for manipulating expressions. Because it's designed to manipulate expressions, the code in the Sym language is built from Sym expressions rather than text.
- Functions
  A function is an object definition that includes clauses its Implementation section.
- Pattern Matching - Overview
  The purpose of a pattern match is to bind an expression to a variable during the execution of code. Most commonly, the expressions that become the values bound variables are the parameters passed into a function call.
  - Objects Used in Pattern Matching
    There are a number of objects used in the pattern matching process, depending on what type of match is required.
    - The Match Object
      The Match object is one of the most commonly used match objects. It matches an expression to a pattern and generates one or more lists of variable bindings.
    - The MatchParameters Object
      The MatchParameters object is used to match to the parameters passed into functions.
    - The MatchIn Object
      The MatchIn object is designed to allow creating pattern match expressions that will generate a list of bindings for each of the parameters of an expression. Among other uses, the MatchIn object provides a way to iterate over a list.
    - The MatchInIndexed Object
      The MatchInIndexed object works the same way as the MatchIn object except that the binding includes both the term and the index of the term in the parent expression.
    - The And, Or, and Not Objects for PatternMatching
      There are two sets of And, Or, and Not objects defined. The first set represent the conventional Boolean objects. The second set is in the Code namespace and are used with pattern matching. These objects can take objects of type Boolean or PatternMatch as parameters. When the And, Or, and Not in the Code namespace contain multiple match objects, they build the appropriate cumulative binding list as the match expressions are executed.
    - The ListVariable Objects
      The ListVariable objects are typically used in patterns where the parent object of the ListVariable is a monoid. Their purpose is to allow matching against a expressions with a variable number of parameters. Patterns with list variables can be used to find a particular pattern anywhere in the list of parameters of an expression.
    - The ListVariableIndexed Object
      The ListVariable object is used to bind to an arbitrary number of terms from a list object. The ListVariableIndexed object is similar, but is designed to do one of two things. It can be used to bind to a specified number of parameters of an object or it can be used to return the number of parameters that were bound
    - The ParameterList Object
      The ParameterList object is used in the processing of clauses, it was introduced to allow list variables and patterns that include list variables to be used on expressions that aren't monoids. It has two applications. First, it allows matching against any object that has a list of parameters. Second it allows putting a list of parameters back into any object.
  - Pattern Matching to Monoids
    In the pattern matching process, there are often cases where we want to create patterns that will match against objects with differing numbers of parameters. Examples include wanting to match against the first parameter of any object that has parameters or matching to any Add object that contains a parameter that has a particular pattern (like "a/b", for example).
  - Pattern Matching on Numbers and Text
    All numbers are represented by the Num object in the Math namespace. A Num object can have a value attached to it. Similarly, strings of text are created using the Text object in the Text namespace. A Text object can optionally have a value attached to it.
  - Pattern Matching to Arrays of Variables
    The purpose of a pattern match is to bind an expression to a variable. There are several advanced ways that variables can be used in this process.
  - Pattern Matching to Null Typed Objects
    The standard pattern matching objects are not designed to handle objects with the Null type. There are pattern matching objects designed to allow you to match to Null typed objects.
- Unit Testing
  Unit tests provide a way to validate that code functions correctly and to detect errors that might be introduced as code is modified. A Unit Test Module contains a list of unit tests. Unit tests can be run individually, all of the tests in a unit test module can be executed at the same time, or using the Unit Test Runner, unit tests from multiple unit test modules can be run simultaneously.
- Exporters
- The Build Process
  The build process converts the clauses in object definitions into a form that can be executed. The build process also runs the type inferencing process on all of the expressions in the document, which speeds up execution of commands and unit tests. At the end of the build process, the rendering of any open modules is updated to reflect any changes to the renderings in object definitions.

The MatchIn Object

The MatchIn object is designed to allow creating pattern match expressions that will generate a list of bindings for each of the parameters of an expression. Among other uses, the MatchIn object provides a way to iterate over a list.

An example will help demostrate how the MatchIn object is used:

This expression will find the first value in the list that is greater than zero, in the example the return value would be 2.

Similarly, we can write a clause that returns a list containing all of the values in the original list that are greater than zero:

The only difference between the first and second examples is that in the second example we are using AssertList instead of Assert for the outer assert. The return value in this case is a list containing 2 and 3.

Testing for properties is another common application for the MatchIn object:

in this clause, the right-hand side is a list of the properties in expr.

MatchIn can be used with expressions other than just lists. The MatchIn object treats whatever object is on the right-hand side as a list of parameters, so we could write a function named GetParameters[f] that contained the clause:

and GetParameters[a + b + c] would return List[a, b, c], GetParameters[a - b] would return List[a, b], and so on.

The MatchIn object can also be used with patterns on the left-hand side other than just a variable, for example:

in this case, each of the parameters of f will be matched to the pattern a/b. If the match succeeds, then the expression on the right-hand side will be generated using the bound variables from the match. If this clause is the only clause in the function MyFn3, then

will return a list containing x and y.