Small snippets of Guile Scheme

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There’s a lot of implicit knowledge among Guile developers. Here I gather some useful snippets I found along the way.

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More useful stuff to get things done in Guile is available in guile-basics and py2guile.

log-expression: print variable name and value

During debugging I often want to display variables by name and value. I used to print name and value by hand, but this quickly becomes tedious:

(define foo 'bar)
(format #t "foo: ~a\n" foo)
;; or
(display 'foo)(display foo)(newline)

Therefore I build me something simpler:

(define-syntax-rule (log-exprs exp ...) (begin (format #t "~a: ~S\n" (quote exp) exp) ...))

Now I can simply log variables like this:

(define foo 'bar)
(log-exprs foo)
;; => foo: bar
(define bar 'baz)
(log-exprs foo bar (list "hello"))
;; foo: bar
;; bar: baz
;; (list hello): ("hello")

Use Guile-yaml to search for the first match in a yaml file

This uses guile-libyaml to parse the yaml file and (ice-9 match) to recursively search within the file.

The example file is demo1.yml from the guile-yaml repo:

--- 
 doe: "a deer, a female deer"
 ray: "a drop of golden sun"
 pi: 3.14159
 xmas: true
 french-hens: 3
 calling-birds: 
   - huey
   - dewey
   - louie
   - fred
 xmas-fifth-day: 
   calling-birds: four
   french-hens: 3
   golden-rings: 5
   partridges: 
     count: 1
     location: "a pear tree"
   turtle-doves: two

I’m searching for the first match to "partridges":

(import (yaml) (ice-9 match))
(define demo (read-yaml-file "demo1.yml"))
(let match-demo ((demo demo))
  (match demo
    ((("partridges" . b) c ...) b)
    (else (if (pair? demo)
              (or (match-demo (cdr demo)) (match-demo (car demo)))
              #f))))
;; => (("count" . "1") ("location" . "a pear tree"))

To use this snippet, start guile as guile -L . in the guile-libyaml repo.

Count occurences of each key in an alist

This was asked by fnstudio in the #guile channel on Freenode.

Given a list of pairs (like xy-coordinates), count how often the first element appears.

(define xydata '((1 . 1)(1 . 2)(1 . 3)(2 . 1)(2 . 2)))

(define (assoc-increment! key alist)
  "Increment the value of the key in the alist, set it to 1 if it does not exist."
  (define res (assoc key alist))
  (assoc-set! alist key (or (and=> (and=> res cdr) 1+) 1)))

(fold assoc-increment! '() (map car xydata))
;; => ((2 . 2) (1 . 3))