What is a lambda (function)?

Question

For a person without a comp-sci background, what is a lambda in the world of Computer Science?

Answer 1

Slightly oversimplified: a lambda function is one that can be passed round to other functions and it's logic accessed.

In C# lambda syntax is often compiled to simple methods in the same way as anonymous delegates, but it can also be broken down and its logic read.

For instance (in C#3):

LinqToSqlContext.Where( 
    row => row.FieldName > 15 );

LinqToSql can read that function (x > 15) and convert it to the actual SQL to execute using expression trees.

The statement above becomes:

select ... from [tablename] 
where [FieldName] > 15      --this line was 'read' from the lambda function

This is different from normal methods or anonymous delegates (which are just compiler magic really) because they cannot be read.

Not all methods in C# that use lambda syntax can be compiled to expression trees (i.e. actual lambda functions). For instance:

LinqToSqlContext.Where( 
    row => SomeComplexCheck( row.FieldName ) );

Now the expression tree cannot be read - SomeComplexCheck cannot be broken down. The SQL statement will execute without the where, and every row in the data will be put through SomeComplexCheck.

Lambda functions should not be confused with anonymous methods. For instance:

LinqToSqlContext.Where( 
    delegate ( DataRow row ) { 
        return row.FieldName > 15; 
    } );

This also has an 'inline' function, but this time it's just compiler magic - the C# compiler will split this out to a new instance method with an autogenerated name.

Anonymous methods can't be read, and so the logic can't be translated out as it can for lambda functions.