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https://github.com/Jaffe-/lispc

A simple Lisp interpreter in C
https://github.com/Jaffe-/lispc

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A simple Lisp interpreter in C

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lispc

=====



A simple Lisp interpreter written in C. It implements the most basic Lisp special forms (called operators here) as well as a small number of primitive procedures. It is lexically scoped and supports closures. 



## Building



lispc should build out of the box on any system. Using gcc, simply do



> `gcc *.c -o lisp -std=c99`



Running the resulting executable will start the lispc interpreter. It does not parse files at the moment, and the REPL is very simple.



## Language



lispc is a minimalistic Lisp language. Expressions in lispc are either symbols, procedures, numbers or lists. Symbols are used as names in variable bindings, or can be used as identifiers for other purposes. A symbol evaluates to the value it is bound to. Numbers and procedures evaluate to themselves. List evaluation follows two basic rules: If the first element is a symbol and the symbol denotes one of the *operators*, evaluation rules for the particular operator is followed. If the first element does not denote one of the operators, the list expression is a function call. Every element in the list is evaluated, and the first element, if resulting in a procedure, is called with the rest of the elements as arguments.



Some examples:



Numbers evaluate to themselves:



    LISP> 1234

    => 1234



`def` is an operator, so the following list expression will be an operator call:



    LISP> (def k 2)

    => K



The symbol `k` will after that call refer to the value 2:



    LISP> k

    => 2



A list where the first element is not an operator is a function call:



    LISP> (+ 1 (* 1 2) 3)

    => 6



### Operators



#### `\` (lambda)



`(\ (a1 a2 ...) exp)` constructs a function whose formal parameters are `a1`, `a2` and so on, and whose expression is `exp`. 



Example:



    (\ (x y z) (+ x y z))           ; a function which takes three arguments and sums them

    ((\ (x y z) (+ x y z)) 1 2 3)   ; create the described procedure and call it with arguments 1, 2 and 3



#### `def` 



`(def a b)` introduces a new *variable binding* where the symbol `a` is bound tothe value `b`.



Example:



    (def a 5)                       ; define a to be 5

    (def square (\ (x) (* x x)))    ; set square to be the function which squares its argument



#### `if` 



`(if test true-exp false-exp)` evaluates `test`. If the result is non-`NIL`, `true-exp` is evaluated. Otherwise, `false-exp` is evaluated. 



Example:



    (if (= x 2) (+ x 1) 2)          ; evaluates (+ x 1) if x is equal to 2, or evaluates 2 if not



#### `'` (quote)



`(' exp)` simply evaluates to `exp`. The parser will convert expressions of the form `'x` to `(' x)`.



Example:



    LISP> (' k)

    => K

    LISP> 'k

    => K



#### `set!`



`(set! a b)` changes the value what `a` refers to, to the value `b`. 



#### `let`



`(let ((a1 b1) (a2 b2) ...) exp)` introduces *local bindings* of the symbol `a1` to the value `b1`, the symbol `a2` to the value `b2`, and so on. With these bindings, the `exp` expression is evaluated. The bindings only exist during the evaluation of `exp`. Already existant bindings outside the `let` scope will be shadowed by these bindings.



Example:



    (let ((x 2) (y 3))

      (+ x y))                ; x and y are only bound in this expression



#### `do` or `:` 



`(do exp1 exp2 ...)` evaluates the expressions `exp1`, `exp2`, ... in order, and uses the last evaluated value as its value.



Example:



    (do (print 'hi)

      (print 'there) 

      1234)



This will print "HI THERE" on the screen, and the expression will evaluate to 1234.



### Primitives



### Generic functions



The standard library defines the functions `new-generic` and `implement` for using generic functions.



`(new-generic name)` creates a new generic function called `name` and returns a procedure which can call it.



`(implement name fn type)` defines a specific implementation of the generic function. `name` is the symbol of the generic function, `fn` is the implementing procedure, and `type` is a list or a symbol denoting the argument types the implementation handles. If `type` is a single symbol, the implementation is assumed to take a variable number of arguments, and every argument has to match this type. If `type` is a list, its items denote the argument types in order. 



#### Example



The function + might be useful to overload for certain types. The standard library implements + for integers as follows:



    (def + (new-generic '+))

    (implement '+ _+ 'integer)



Here, `_+` denotes the built in primitive procedure for adding integers. The symbolic type argument makes this implementation accept a variable number of arguments, all of integer type. 



To add a new implementation, for example for a type `matrix`, we can do



    (implement '+ add-matrix '(matrix matrix))

    

Here, the type argument is a list of two elements, meaning that this implementation will take two arguments, both of type `matrix`. The function `add-matrix` must be a function taking at least two such arguments.