Some dynamic languages offer an eval function. This function takes a string parameter containing code in the language and executes it. If this code stands for an expression, the resulting value is returned. However, Erik Meijer and Peter Drayton suggests that programmers "use eval as a poor man's substitute for higher-order functions."
A type or object system can typically be modified during runtime in a dynamic language. This can mean generating new objects from a runtime definition or based on mixins of existing types or objects. This can also refer to changing the inheritance or type tree, and thus altering the way that existing types behave.
Reflection
is common in many dynamic languages, and typically involves analysis of the types and metadata of generic or polymorphic data. It can, however, also include full evaluation and modification of a program's code as data, such as the features that Lisp provides in analyzing S-expressions.
Macros
A limited number of dynamic programming languages provide features which combine code introspection and eval in a feature called macros. Most programmers today who are aware of the term macro have encountered them in C or C++, where they are a static feature which is built in a small subset of the language, and are capable only of string substitutions on the text of the program. In dynamic languages, however, they provide access to the inner workings of the compiler, and full access to the interpreter, virtual machine, or runtime, allowing the definition of language-like constructs which can optimize code or modify the syntax or grammar of the language. Assembly, C, C++, early Java, and Fortran do not generally fit into this category.
The example shows how a function can be modified at runtime from computed source code ; the source code is stored as data in a variable CL-USER > ))
BEST-GUESS-FORMULA*
; a function is created from the code and compiled at runtime, the function is available under the name best-guess CL-USER >
; the function can be called CL-USER > 265.225 ; the source code might be improved at runtime CL-USER > ) ; a new version of the function is being compiled CL-USER >
; the next call will call the new function, a feature of late binding CL-USER > 16.28573
Object runtime alteration
This example shows how an existing instance can be changed to include a new slot when its class changes and that an existing method can be replaced with a new version. ; a person class. The person has a name. CL-USER > ))
; a custom printing method for the objects of class person CL-USER >
))
; one example person instance CL-USER >
; the class person gets a second slot. It then has the slots name and age. CL-USER > ))
; updating the method to print the object CL-USER >
)))
; the existing object has now changed, it has an additional slot and a new print method CL-USER > *person-1*
; we can set the new age slot of instance CL-USER > 25 ; the object has been updated CL-USER > *person-1*
Assembling of code at runtime based on the class of instances
In the next example, the class person gets a new superclass. The print method gets redefined such that it assembles several methods into the effective method. The effective method gets assembled based on the class of the argument and the at runtime available and applicable methods. ; the class person CL-USER > ))
; a person just prints its name CL-USER >
))
; a person instance CL-USER >
PERSON-1*
; displaying a person instance CL-USER > *person-1*
; now redefining the print method to be extensible ; the around method creates the context for the print method and it calls the next method CL-USER >
))
; the primary method prints the name CL-USER > )
; a new class id-mixin provides an id CL-USER > ))
; the print method just prints the value of the id slot CL-USER > )
; now we redefine the class person to include the mixin id-mixin CL-USER 241 > ))
; the existing instance *person-1* now has a new slot and we set it to 42 CL-USER 242 > 42 ; displaying the object again. The print-object function now has an effective method, which calls three methods: an around method, the primary method and the after method. CL-USER 243 > *person-1*
Examples
Popular dynamic programming languages include JavaScript, Python, Ruby, PHP, Lua and Perl. The following are generally considered dynamic languages:
