What type of language is Scheme?

Scheme is a progeny of the Lisp family programming languages; therefore, it has a lot in common. Ideally, Scheme is a functional programming language with a straightforward syntax that is based on the S-expression. Further, it uses parenthesized lists in which its arguments follow a prefix operator. So, scheme programs are made up of sequences of nested lists. 

Click here to discover more about how Scheme is important in the grander scheme of things.

The lists make the central data structures, giving a closer equivalence between data formats and the source code. 

While its syntaxes are easy to read, they cannot compare to those of Ruby and Python. However, Scheme differs in highly expressive language, unlike Assembly programming languages. Scheme works on data structures such as tuples, strings, characters, numerical parts, and vectors. Its rich data set makes it a more versatile language. 

Distinguishing features

The following features distinguish Scheme from other programming languages:

Minimalism

The simplicity of Scheme makes it easier to implement than comparable programming languages. The use of lambda calculus to get the most of the language’s syntax from primitive forms is thought to be the primary reason why Scheme is easy to implement. 

Lexical scope

Scheme is lexically scoped, unlike earlier forms of Lisp. That implies you can analyze all possible variable bindings within a program unit through reading the program unit’s text without considering the context of its calling. This is far from dynamic scope, a characteristic or earlier Lisp languages.  

Lambda calculus

Lambda calculus is the keyword for introducing procedures and influencing the development of functional programming techniques that involve high-order functions in Lisp. 

A lambda system contains axioms and a complete calculation rule. It analyses using mathematical logic and tools, and calculations are a directional deduction. The lambda calculus syntax follows the recursive expressions from x, y, z, …, parentheses, spaces, the period, and the symbol λ. 

Block structure

The block structure design of Scheme is an inheritance from previous languages, particularly ALGOL. Three binding constructs implement the blocks in Scheme. The constructs include let, let*, and letrec. 

The program allows the nesting of blocks to create complex structures depend on what the programmer wants. The advantage of using block structuring for creating local bindings eliminates the risk of namespace collision. 

Proper tail recursion

There is an iteration construct in Scheme. However, it is more idiomatic to use tail recursion to express iteration. To optimize tail calls, programmers need the standard-conforming implementation to support unlimited active tail calls, i.e., proper tail recursion. This makes it safer to develop iterative algorithms using recursive structures, which are more intuitive. 

First-class continuations

Continuations are first-class objects within Scheme. The procedure is call-with-current-continuation or call/cc. Its function is to capture the current continuation – it packs it up as an escape procedure bound to formal arguments in a process that the programmer provides. 

With first-class continuations, programmers can create non-local constructs like coroutines, iterators, and backtracking. Further, they can emulate behaviors of return statements in imperative programming languages. 

These features point out the efficiency and power of Scheme. The language comes with multiple compilers, which are extremely fast and efficient. As such, any program you write runs as fast s those written in low-level languages.  

Scheme programming language history

The attempt to comprehend Carl Hewitt’s Actor model is what gave rise to Scheme. Initially, Scheme was referred to as a schemer, and its current name came about as a result of Scheme’s author using the ITS operating system. This system limits the filenames to only two components, each of not more than six characters. 

The development of earlier Lisp members in the 20th century was a great inspiration to the creation of Scheme. The language designers Guy L. Steele and Gerald Jay Sussman released an influential series of AI Memos – Lambda Papers between 1975 and 1980 at the Massachusetts Institute of Technology (MIT). This release popularised the language, promoted the standardization of the language from 1990. 

The Actor model – the birth of Scheme

Carl Hewitt has an ambitious project – Planner Project, and in 1972 alongside his students, he invented the Actor model. This was a computation method that was to act as a solution to problems that arose during Planner’s implementation. Planner-73, also known as PLASMA, was developed as a partial implementation of the Actor.

However, Steele and Sussman opted to implement another version of the Actor model in Tiny Lisp – this was their own model that they develop on MACLISP. It is meant to give a better understanding of the model. This model gave them a solid foundation, and thus, they began to create a mechanism for developing actors and sending messages. 

PLASMA used lexical scope, which was similar to lambda calculus. Therefore, Sussman and Steele decided to model Actors in the lambda calculus and named the system Schemer, but later changed the name to Scheme. The name changed was prompted by the condition to fit the ITS file system six-character limit on the DEC PDP-10. 

The duo then concluded Actors but closures that never return; instead, it invokes a continuation. So, they decided that the Actor and closure were identical concept options – in the context of their research. 

Further, they removed the code’s redundant part, resulting in small, effective, and capable code, a Lisp dialect. Scheme had a hairy control structure, which Hewitt criticized, terming it as primitive. 

Sussman and Steele, in 1998, observed that schemes minimalism wasn’t a conscious design aim; instead, it was an unintended outcome of the design process. They intended to create a complicated program but ended up making a simple program that met all their goals. Ideally, the lambda calculus is the core, providing robust and expressive powers within this programming language. 

2003 saw the beginning of the standardization process and is aimed at developing the R6RS standard by 2006. The R6RS came with a standard module system and could separate the core language and the libraries. The project wasn’t successful; however, with multiple drafts, the final product was R5.97RS, ratified on 28th August 2007. 

R6RS standard brought many changes to the program and was considered a shift from the minimalist design philosophy. In 2009 august, the language was broken into two: a minimalist version and a modern version. 

Click here and get up to speed with the latest from Scheme.

Scheme interpreter

Interpreters are essential parts of particular programming languages. An interpreter reads the source texts, evaluates the text, and executes it. So, an interpreter is a computer program utilized for executing program instructions written in high-level programming languages.

An interpreter converts high-level languages into intermediate programs before executing them. However, at times it parses high-level source code before directly performing the commands. These operations are done line by line. 

Though effective, a pure interpreter works a bit slower considering the intensive process of analyzing text character strings to determine what they mean. Since it must identify and analyze all text lines one at a time, the process is time-consuming. 

Scheme has a number of inbuilt interpreters, which give it a versatile profile. Real interpreters lie between compilers and pure interpreters. They just read through the source code once and then translate it into an intermediate form, which is easier to manipulate – it just is a type of data structure.

A real interpreter doesn’t analyze source code strings; instead, it goes through data structures, representing the source text but more conveniently and quickly. Simply put, they perform analysis once and convert source text into a data structure. 

The standard type of scheme interpreters include:

• Bigloo: this is a Lisp dialect and a scheme implementation. Its prominent role is to enable Scheme-based programming style in cases where C++/C is needed. 
• Chez Scheme: it’s a scheme implementation that utilizes incremental native-code compiler to generate native binary files for PowerPC, SPARC and x86, processor architectures. It supports the R6RS standard from version 7.9.1.
• Gambit: this Scheme variant consists of both scheme interpreter and compiler. It can compile Scheme into language C, making it a cross-platform software. 
• GNU Guile: it comes with multiple extensions for various programming languages. For instance, libguile allows embedding Scheme into other languages. 
• JScheme: it complies with the R4RS Scheme standard that interfaces with Java.
• Scheme 48: this dialect of Scheme emits bytecode, and it features a foreign function interface to call a function from the C language.  
• TinyScheme: it features a lightweight Scheme interpreter of R5RS standard. It works as an embedded scripting interpreter.

Others include MIT/GNU Scheme, Pvts, STklos, SIOD, Ypsilon, and LispMe. 

Why should you use a scheme interpreter?

• An interpreter can bring to life some of the convenient scheme features. 
• Command interpreters help programmers develop decent codes due to their influence on usability and system power. 
• Scheme interpreter help to clarify issue within a language

What is the difference between a compiled and an interpreted program?

Programs are just sets of instructions to perform various tasks, including sending messages, adding numbers, etc. So, they form communication within a computer. 

But a computer understands numbers only (binary language); so, for humans to talk to a computer, they require a translator, which is interpreter or compiler. The compilers and interpreters convert human-readable code into machine-readable code. Humans can only comprehend high-level programming languages, customarily called source codes. 

The main difference between compilers and interpreters is in the results of the procedure of compiling and interpreting. An interpreter generates results from the programs via an interpreter, which reads then executes the given code. 

Compiled programs 

The compiler typically generates a program in a compiled language. The compilers architecture assembler turns the resultant program into a binary code. Ideally, the target machine directly translates the program. 

Note that there are different kinds of Assembly languages – they differ based on the architecture. That means that compiled programs run on a computer having the same system structure as the computer that assembled them. 

By now, you know that compiled programs are not human readable. They basically are within architecture-specific languages. Their creation is not straightforward as it involves several steps. 

Firstly, the programmer writes the code either using a common text editor or a development tool using one of the many computer languages. For complex programs, it wise that the code pieces be spread across different files. After writing the code successfully, the programmer compiles it. The process involves linking the modules, sorting the codes, and then translating the code into understandable computer code, i.e., machine code. 

Remember, a compiled program works on a platform similar to the one created – for instance, programs developed on HP-UX work on a Mac OS computer. Despite this limitation, compiled languages are much faster and effective than programming languages running through an interpreter. The fantastic thing is that you can recompile the programs to make them run on multiple platforms. Languages that can produce compiled languages are Fortran, C, and COBOL.

Interpreted program

Here, the source code is the program, and such programs are typically called scripts. Scripts need an interpreter to parse the commands within the programs and then execute the programs. Interpreters run through the program line-wise, executing each command. The advantage of this is that the programmers can change the codes on the code.

There different kinds of code interpreters and they work differently. For instance, Unix shells (csh, sh, ksh) can read and then execute each command immediately. However, other compilers like Perl first analyze the whole script prior to dispatching the machine language commands. 

A script is highly portable – it can be used on a computer with the right interpreter. It runs the entire program without changing anything. While it is an advantage, it may confer some disadvantages too – the programs won’t run if there is no interpreter. 

Though interpreted programs run much slower than their compiled counterparts, debugging and revising them is very easy. 

Other examples include Python and JavaScript.

Scheme vs. Python

Python and Scheme are both powerful programming languages – not all of them are used to carry out the same operation. While Python is interpreted, Scheme has characteristics of both interpreted and compiled languages. 

Most programmers prefer using Scheme/List because you can write a functional code very fast than in most other high-level languages such as Python. However, Python is much more practical and faster than Scheme, thanks to its lightweight design. 

Currently, the first-class procedure and the idea of Lisp have invaded the mainstream. That still shows how relevant Scheme and List are 50 years down the line. While Python offers the most advantages to modern programmers, its developers could not a fully baked version of lambda in Python.

Scheme language tutorial

It’s true; Scheme is one of the most powerful programming languages. It works with both compilers and interpreters; thus, offering programmers immense flexibility. With this program, you can create almost anything you need – ranging from financial analysis to different computer programs. 

While that may sound encouraging, this functional, general-purpose programming language isn’t beginner-friendly like other high-level languages. But the learning curve isn’t very much steep. If you have to deal with any LISP programs, the Scheme becomes easy because they share almost a similar skeleton. 

There are many tutorials to guide you through Scheme from beginner to expert. Understanding the details of the Scheme takes a little while. So, practicing how to use the program alongside understating the different components is vital in your learning journey. 

Beginners have to start from the first page – the introduction. 

Scheme Introduction: This section introduces the learner to Scheme. Typically, it deals with functional core and a few simple input/output operations. 

Scheme Programs Structure

Here, you’ll learn about a set of function definitions within Scheme program – no imposed structure or the primary function. Besides, it is possible to nest the primary function definition and execution is by evaluating an expression after submission. 

Expression and functions are represented as: 

(function_name arguments)

This representation is a shift from the ordinary mathematical syntax – the function name is inside parentheses. Besides, spaces separate the arguments instead of commas. 

The Syntax

Scheme and lambda calculus are syntactically close. So, the Scheme Syntax appears as:

E for Expressions

I for variable/ Identifiers

K for Constants

So, you get: 

E ::= K | I | (E_0 E^*) | (lambda (I^*) E2) | (define I E’)

A star sign that follows a syntactic category means more repetitions of an element of a category or zero. 

Types

Scheme provides atoms or constants, including numbers, empty lists (), Boolean, and strings. Atoms represent variable and function names, while a list is a set of elements in order – it may contain other lists of atoms. As in Lisp, enclose lists with parenthesis.

There are different categories of Types – Simple Types, Composite Types, and Type Predicates.

Arithmetic Operations, Functions and Numbers 

There are data types in scheme, and it includes rational, real, the integer, and complex numbers. Its written as {\tt number} – for example, you can write (+ 4 5 2 7 5 2) as ( 4 + 5 + 2 + 7 + 5 + 2 \). 

This section also covers arithmetic operators, lists, Boolean expressions, and different operators, i.e., logical and relational. 

Functions

Here, you’ll learn how to write functions such as definition expression (define {\em id exp}), and lambda expressions (lambda ({\em id…}) {\em exp} ).

Nested Definitions

You can perform local definition by nesting the definitions. There various functions that introduce definitions, e.g., {\tt let}, {\tt letrec} and {\tt let*} for loacls definitions. 

Input-Output Expressions

The input in Scheme doesn’t pass as a parameter but is obtained due to successive evaluation using inbuilt functions. 

These sections, alongside high order function, offer insights to anyone interested in learning Scheme. 

Conclusion

There is no clear-cut placement of Scheme into either a compiled or interpreted language because it massively supports both. Scheme has a lot of implementations that allow programmers to interpret it much easy. But then again allows compilation in an even more straightforward manner. 

Keep in mind that Scheme has a unique range of tools and features, making it compatible with various implementations. However, some are designed explicitly for Scheme. Lots of scheme compilers and interpreters exist, and all of them work differently. A scheme interpreter examines and dispatches expressions. Besides, it performs the actual operations that the program specifies. 

The most recommended interpreter in Scheme is the 

DrRacket, thanks to its favorable environment and support for multiple dialects in Scheme. Additionally, it gives good debugging and failure information.

Another example is Gambit-c is an influential interpreter and compiler, and its versatility can turn Scheme into a cross-platform language. 

Compilers are unique interpreters that help programmers to execute codes. In most cases, programmers will use offline compilers and interpreters to help them analyze and execute codes. However, advanced technology has brought online interpreters and compilers – programmers can write and test codes online. So, no need to download software.