-- mode: org --

+TITLE: Exercism Scheme Track

+AUTHOR: Jason Lewis

[] []

Exercisms in scheme.

** Overview

Welcome to the home of the exercism scheme track! Many parts of the track are specified inside scheme and then output in various formats. For example, exercism wants a =config.json= file, which is actually specified in [[file:config/track.ss][config/track.ss]]. Everything including the exercise implementations, and the documentation is also done from scheme. In general, the way to modify the track is to edit something in the [[file:input/][input/]] directory. Exercism problem implementations live in [[file:input/exercises/][input/exercises/]]. Student facing documentation (like resources and links) is built from files in [[file:input/docs/][input/docs/]].

** Setup

To work on the scheme track you'll need [[https://cisco.github.io/ChezScheme/][ChezScheme]] and [[https://www.gnu.org/software/guile/][GNU
Guile]]. The goal is for exercises to be implemented such that they
work in either scheme. Each exercise has tests and an example
solution and, as part of the build process, it is checked that
these tests pass in both schemes.

There is a small json library (borrowed from [[https://github.com/weinholt/packrat][packrat]]) in
=code/json.sls=, which relies on some modules from the [[https://github.com/fedeinthemix/chez-srfi][srfi]]
collection. ChezScheme finds libraries through an environment
variable =CHEZSCHEMELIBDIRS=. So, you'll need to make sure the
location of your downloaded srfi is added to
=CHEZSCHEMELIBDIRS=. Instructions for installing the srfi
collection can be found at [[https://github.com/fedeinthemix/chez-srfi/blob/master/srfi/INSTALL.chez][srfi/INSTALL.chez]]. If you're using
either the [[https://guix.gnu.org/][guix]] or [[https://nixos.org/nix/][nix]] package managers, you can find the srfis
pre-packaged.

** Working on the Scheme track

To work the track, you should start by heading to this directory
and firing up a Scheme REPL. Enter =(load "load.ss")=, which
orchestrates loading the appropriate files.

Exercises are typically specified in [[https://github.com/exercism/problem-specifications][problem-specifications]], a
repository that has descriptions, versions, and tests described
through JSON files. The [[file:Makefile][Makefile]] will clone it; the scheme in
[[file:code/][code/]] will read problem information from that JSON. It is,
however, also possible to define novel exercises if you wish.

*** Requirements

Exercises have a few required parts, taking the exercise
[[file:input/exercises/pascals-triangle/][input/exercises/pascals-triangle/]] as an example.
+ The [[file:input/exercises/pascals-triangle/example.scm][example.scm]] file which is an example solution that must pass
  the tests. It must also use only =r6rs= scheme features so that
  it can run in both Guile and Chez scheme.
+ A stub file [[file:input/exercises/pascals-triangle/pascals-triangle.scm][pascals-triangle.scm]] usually named after the problem
  that the student will be given to write their solultion.
+ The [[file:input/exercises/pascals-triangle/test.ss][test.ss]] which builds the test cases as well as problem
  metadata (such as problem versions, read from the
  specifications) and extra documentation (if needed). The crucial
  part of this file is a call to ~put-problem!~, a procedure
  defined in [[file:code/track.ss][code/track.ss]] that defines the problem whose name is
  a symbol (eg ='pascals-triangle=) and an association list with
  the following form

+begin_src scheme :exports code

(let ((spec (get-test-specification 'pascals-triangle))) (put-problem! 'pascals-triangle `((test . ,(spec->tests spec)) (stubs pascals-triangle) (version . ,(lookup 'version spec)) (skeleton . "pascals-triangle.scm") (solution . "example.scm") (markdown . ,(splice-exercism 'pascals-triangle)))))

+end_src

The tests are parsed in a fairly horrible and ad-hoc way:

+begin_src scheme :exports code

(define (spec->tests spec) (map parse-test (lookup 'cases (car (lookup 'cases spec)))))

+end_src

due to the fairly irregular problem specifications. =parse-test=
is defined:

+begin_src scheme :exports code

(define (parse-test test) `(test-success ,(lookup 'description test) equal? pascals-triangle '(,(cdar (lookup 'input test))) ',(lookup 'expected test)))

+end_src

=pascals-triangle= has a fairly friendly specification. An example
of a less straightforward one from [[file:input/exercises/change/test.ss][change]] is the following:

+begin_src scheme

(define (parse-test test) (let ((expected (lookup 'expected test)) (input (lookup 'input test))) (if (or (null? expected) (number? (car expected))) (test-success ,(lookup 'description test) (lambda (out expected) (equal? (list-sort < out) (list-sort < expected))) change '(,(lookup 'target input) ,(lookup 'coins input)) ',expected) (test-error ,(lookup 'description test) change '(,(lookup 'target input) ,(lookup 'coins input))))))

+end_src

There are test cases expected to fail and a more elaborate passing
predicate to allow students to return answers in any order.

Final requirement:

 + Add the problem to the configuration expression in
  [[file:config/track.ss][config/track.ss]]. You need to provide a uuid, which can be
  generated from the scheme repl by calling =(configlet-uuid)=
  (which is just a wrapper for the configlet binary). Finally,
  include the problem in the list of implementations in the
  [[https://github.com/exercism/scheme/blob/master/Makefile][Makefile]].

When [[file:Makefile][building]] the track, the makefile reads each exercise
directory and uses the =test.ss= files to generate the [[file:exercises/pascals-triangle/test.scm][test.scm]]
files that the students use. The procedure =spec->tests= reads the
parsed specification json and turns that into runnable scheme
tests. The json is parsed through a procedure
~get-test-specification~.

Basically, =test.ss= defines each problem as an instance of an
exercism problem, having the parts described above, which are used
to output the files that the students get.

*** Extras

Helpful and useful procedures include:

 - To start a new exercise, use =(stub-exercism 'change)=. This
   creates rough stubs for =test.ss=, =change.scm=, and
   =example.scm= in the directory [[file:input/exercises/change/][input/exercises/change/]].
 - =(verify-exercism 'change)= checks that the solution passes
   generated test suite. It ought to work under both =Guile= and
   =Chez=, with =(rnrs)= as the imported library.

*** Adding problems not in exercism/problem-specifications

New problems from outside of the specifications are welcome as
well. Develop them as described above in the =code/exercises=
directory, following pretty much the same process as above. The
main difference is that instead of parsing test cases from the
problem-specifications repository, you'll write them by hand in a
=test.ss= file.

** Scheme icon

The Scheme logo was created by https://en.wikipedia.org/wiki/User:Matthias.f and released under the Creative Commons Attribution-Share Alike 3.0 Unported license. We adapted the logo, creating a pink/black version to use on Exercism.