Lazar

Toy library for R6RS Scheme

Code examples shown here

Overview

Mostly fiddling around trying to learn the ropes, but I have a soft spot for this kind of syntax, so I'll be probably rolling it in any of my real projects, if I ever get to them.

So, purely for fucking around and teaching myself how to handle macros, create libraries (I know the R7RS lib spec is a bit different, but not different enough to undo all I picked up.)

Libraries

• (lazar)
  • (lazar fn-lambda)
    • SML-reminiscient anonymous (lambda) function.
  • (lazar est)
    • OCaml-like let / let rec-syntax for let* / letrec*
  • (lazar case-with)
    • SML-like syntax sugared cond that should do pattern matching too somewhere in the future.
  • (lazar basic-syntax)
    • math / Coq / Haskell-inspired notation for some miscellaneous functions and macros.
  • (lazar cxr)
    • cadadr-style abbreviations to hd and tl format (ie. h:hd => caar)
  • (lazar contracts)
    • SML-inspired rework of Linus Björnstam's contracts for function definition.

"Features"

• SML-like syntax

car    := hd
cdr    := tl
cadr   := h:tl
cadar  := h:t:hd

cons   := ::
cons*  := ::*

• unicode logic symbols, and some of their ascii equivalents to use

#t  := ⊤, true
#f  := ⊥, false
and := /\, ∧, ⋀
or  := \/, ∨, ⋁
not := ¬

( ... )

'()       := Ø
(not '()) := ¯

• define with contracts (with define/c)

(define/c (<proc-name> ((<param> : <type>) ...) -> <return-type>) body ... )

• and for single parameter/variable-cases:

(define/c (<proc-name> (<param> : <type>) -> <return-type>) body ... )

• Types to contract include at the very least:

  • any boolean (easy to construct your own type). Example of this seen below.

  • ListOf x

  • VecOf x

  • Any, a', α

    • etc.

  • Defining new contract/type/predicate:

(define (Natural n)
  (and (exact? n) (ℤ n) (>= n 0)))

• sugared cond, much like the one found in Standard ML.
  • nb. => is purely sugar and case-with functions just as well without it.

(case-with || <clause> => <expr>
           || <clause> => <expr> ...
           || else     => <expr>)

• Example of define/c and case-with on a Little Schemer -fame function:

(define/c (rember ((x : α) (ys : List)) -> List)
  (case-with || (Ø? ys)       => Ø
             || (= x (hd ys)) => (tl ys)
             || else          => (:: (hd ys) (rember x (tl ys)))))

• SML-inspired lambda notation.
  • Without the auxilliary =>, it handles one variable, marked with underscore.

((λ x y z => (+ x y z)) 20 19 3)

;; indices should start at 1 goddamnit
(define/c (ι (n : ℕ) -> List)
  (map (λ (+ _ 1)) (iota n)))

• est / ε works like a quick let* and letrec*-closure.
  • rec marking the following definition as a recursable from the closure.

(define/c (rev (xs : List) -> List)
  (ε rec aux
     (λ x y =>
        (case-with || (Ø? x) => y
                   || else   => (aux (tl x) (:: (hd x) y))))
     in (aux xs Ø)))

Funnily enough Emacs handles indentation of all this relatively well

Thanks to

r/scheme

Linus Björnstam

#scheme @ freenode