Pref

(A simple project to get comfortable with my current Haskell dev environment, cabal etc.)

Pref is a lazy, lisp-like programming language

---

Usage

Installing Haskell & Cabal

To use Pref, you need to have a minimal Haskell dev-environment installed.

On *nix systems, checkout ghcup to install both Haskell and Cabal

Interpreting Pref programs

With cabal-install installed, you can execute the following commands to evaluate a Pref program:

> cabal build all
> cabal exec pref *path*

where path points to a file containing a Pref program (also take a look at the .github CI/CD setup for running tests)

---

Syntax/Grammar

• d ::= (define x e)
• e ::= x
  | (e e ...)
  | (lambda (x ...) e)
  | (let ([x e] ...) e)
  | (if e e e)
  | empty
  | c | B
• x ::= variable
• c ::= integer | string | b
• b ::= #t | #f
• B ::= + | - | * | / | cons | car | cdr | empty? | zero? | string-append | fix | and | or | not

You can also use braces or brackets instead of parenthesis where needed.

Variable

A variable identifier in Pref can be alphanumeric, as long as there is at least one non-numeric character. It can also contain periods as long as there is at least one non-numeric character. The following characters are not allowed:

• open/close parenthesis, braces, brackets, or angles
• single/double quotes
• comma, quasiquote (`), #, :, or ;

---

Data Types

Pref comes with the following built-in data types:

• Integers
• Strings
• Booleans
• Lists

---

define

The define grammar can be used to define top-level constants and functions.

For top-level functions, define is used as follows:

(define f (lambda (x ...) e))

Currently, define can only be used at the top level.

---

TODOs

• Allow user to choose an evaluation strategy by passing in certain flags.
• Data Types: Allow user to define their own data type and (maybe) using pattern matching to work with the data.
• Refactor: Pref had a rough start which is why define can only be used at the top-level. This should be allowed where
  a user expects to use let.
• Error Locations: Both parser and the interpreter should indicate the location where the error occured.
• Compiler: compile Pref to x86.
• Infix Grammar: Try to use the same interpreter on an infix version of Pref (I guess it will be called Infi if that happens).
• Naive Macros: Compile time functions to manipulate the syntax tree.