[bug#49958] [PATCH] More flexibility in opam importer

[Alice BRENON <alice.brenon@ens-lyon.fr>]

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Le Fri, 13 Aug 2021 09:37:43 +0200,
Xinglu Chen <public@yoctocell.xyz> a écrit :

> On Tue, Aug 10 2021, Alice BRENON wrote:
> 
> […]
> > include: +
> > +By default, packages are searched in the official OPAM repository.
> >  This +option, which can be used more than once, lets you add other
> > +repositories where to look for packages.  
> 
> “lets you add other repositories where to look for package” sounds a
> bit weird, maybe
> 
>   lets you add other repositories which will be used to lookup
> packages.
> 
> ?

Ok, as discussed on IRC, trying "lets you add other repositories which
will be searched for packages".


> What happens if I specify an additional repository, and a package
> exists in that repository _and_ the default opam repository?  From
> which repository will the package be imported from?

That is the beauty of it: the repositories are assumed to be passed by
order of preference, defaulting to the official opam repositories only
if packages haven't been found anywhere else. Writing this makes me
realize that indeed, starting with --repo=opam isn't entirely
redundant: it could be used to prevent an otherwise interesting repo
from overriding stuff if opam already provides it (let's assume some
"super-opam" with a couple additional packages, and custom versions of
existing opam packages).

Calling `--repo=super-opam` would use the super-opam versions as soon
as a package exists in super-opam, while `--repo=opam
--repo=super-opam` would take the super-opam versions only when none
exist in opam.

A much simpler use-case would be to locally override only some
packages in a repo, and pass --repo=overriden-repo --repo=normal-repo.

This behaviour relies on the implementation of opam-fetch and how folds
work in guile.

Since in the importer script options are stacked as they are retrieved
from the CLI arguments, and repositories are then just filter-maped from
that list, they end up in a list by reverse order of preference. In
opam->guix-package, 'opam gets push on the top if it's not already
there somewhere. So what we get as input of opam-fetch is a list of
repositories-specs by reverse order of preference. Now fold applies the
accumulator to each item in order, so, last elements has the final say,
i.e. the last elements which yield results in find-latests are
preferred over the earlier elements of the list. This works for the
same reason why `(lambda (l) (fold cons '() l)` will reverse its input
list. It's slightly inefficient because it means all repositories are
searched, in reverse order of preference, but I haven't figured how to
get a lazy fold in guile. Granted, I could have written the recursion
explicitly to get that. Will fix if performance matters.

Also, versions are not compared between repositories, as soon as a repo
provides one version of a given package, the latest of all the versions
this one provides is used in the output, no matter the contents of
other repositories. This is useful to allow "downgrades" by masking
parts of repository which have too recent versions.

So, thanks for your remark, the documentation deserved a clearer
explanation of it.

> […]
> > -(define (latest-version versions)
> > -  "Find the most recent version from a list of versions."
> > -  (fold (lambda (a b) (if (version>? a b) a b)) (car versions)
> > versions)) +(define (get-version-and-file path)
> > +  "Analyse a candidate path and return an list containing
> > information for proper
> > +  version comparison as well as the source path for metadata."
> > +  (and-let* ((metadata-file (string-append path "/opam"))
> > +             (filename (basename path))
> > +             (version (string-join (cdr (string-split filename
> > #\.)) ".")))
> > +    (and (file-exists? metadata-file)
> > +         (eq? 'regular (stat:type (stat metadata-file)))
> > +         (if (string-prefix? "v" version)
> > +             `(V ,(substring version 1) ,metadata-file)
> > +             `(digits ,version ,metadata-file)))))  
> 
> What happens if some other prefix is used, e.g., “release-” or “V-”?
> 

It would get marked as a 'digit. In a previous draft before I started
sending this series of patches, it was called 'R, standing for
"regular", then I thought it was not very meaningful, and, since the
versions were to my knowledge supposed to be either v[0-9]+(\.[0-9]+)*
or [0-9]+(\.[0-9]+)*, I thought I could call that default case "digits"
to clearly indicate what I was trying to refer to. I could change it to
'other if it matters too much, but the important thing here is that we
distinguish between v-prefixed (the so-called "janestreet
re-versionning" mentioned inside the implementation of find-latest on
current d87d6d6 master) and other versions because ⬇️

> Also, why not just return the version number and the metadata file; we
> don’t really care about the prefix do we?
> 

yes we do ! the former latest-version finder handled strings, and
dropped this prefix or put it back on the fly, but the logic
implemented was: if there are v-prefixed versions, find the greatest of
them, without the initial "v", if there aren't, just find the greatest
of all versions. This implies that v-prefixed versions are considered
more important and automatically greater than non-prefixed versions, no
matter what the numbers, which is why this information must be kept.

I'm just playing ADTs in guile here, "parsing" the version string only
once to retain a symbolic representation of it that will at first
glance allow to identify the type of version used and access the
relevant digits for comparison. The comparison is used right after:

> > +(define (keep-max-version a b)
> > +  "Version comparison on the lists returned by the previous
> > function taking the
> > +  janestreet re-versioning into account (v-prefixed come first)."
> > +  (match (cons a b)
> > +    ((('V va _) . ('V vb _)) (if (version>? va vb) a b))
> > +    ((('V _ _) . _) a)
> > +    ((_ . ('V _ _)) b)
> > +    ((('digits va _) . ('digits vb _)) (if (version>? va vb) a
> > b)))) 

and used in the reduce in find-latest-version. So keeping this 'V is
what will help janestreet-re-versionned packages "skip the line" by
being automatically greater than any non v-prefixed package (thus,
v0.0.1 is greater than 13.2, which is the current behaviour).

> >  (define (find-latest-version package repository)
> >    "Get the latest version of a package as described in the given
> > repository."
> > -  (let* ((dir (string-append repository "/packages/" package))
> > -         (versions (scandir dir (lambda (name) (not
> > (string-prefix? "." name))))))
> > -    (if versions
> > -      (let ((versions (map
> > -                        (lambda (dir)
> > -                          (string-join (cdr (string-split dir
> > #\.)) "."))
> > -                        versions)))
> > -        ;; Workaround for janestreet re-versionning
> > -        (let ((v-versions (filter (lambda (version)
> > (string-prefix? "v" version)) versions)))
> > -          (if (null? v-versions)
> > -            (latest-version versions)
> > -            (string-append "v" (latest-version (map (lambda
> > (version) (substring version 1)) v-versions))))))
> > -      (begin
> > -        (format #t (G_ "Package not found in opam repository:
> > ~a~%") package)
> > -        #f))))
> > +  (let ((packages (string-append repository "/packages"))
> > +        (filter (make-regexp (string-append "^" package "\\."))))
> > +    (reduce keep-max-version #f
> > +            (filter-map
> > +             get-version-and-file
> > +             (find-files packages filter #:directories? #t)))))
> >  
> […]
> > +            (filter-map get-opam-repository repositories-specs))
> > +      (leave (G_ "Package '~a' not found~%") name)))  
> 
> Nit: I wouldn’t capitalize “package”, otherwise the error message
> looks like this
> 
>   guix import: error: Package 'equations' not found

a very neat tip, thank you ! : )


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From cde8b2a5d88d89bfea31c86d3ae94d37c1d3c83f Mon Sep 17 00:00:00 2001
From: Alice BRENON <alice.brenon@ens-lyon.fr>
Date: Sat, 7 Aug 2021 19:50:10 +0200
Subject: [PATCH] guix: opam: More flexibility in the importer.

* guix/scripts/import/opam.scm: pass all instances of --repo as a list
  to the importer.
* guix/import/opam.scm (opam-fetch): stop expecting "expanded"
  repositories and call get-opam-repository instead to keep values
  "symbolic" as long as possible and factorize.
  (get-opam-repository): use the same repository source as CLI opam does
  (i.e. HTTP-served index.tar.gz instead of git repositories).
  (find-latest-version): be more flexible on the repositories structure
  instead of expecting packages/PACKAGE-NAME/PACKAGE-NAME.VERSION/.
* tests/opam.scm: update the call to opam->guix-package since repo is
  now expected to be a list and remove the mocked get-opam-repository
  deprecated by the support for local folders by the actual
  implementation.
* doc/guix.texi: document the new semantics and valid arguments for the
  --repo option.
---
 doc/guix.texi                |  30 +++++--
 guix/import/opam.scm         | 158 +++++++++++++++++++++--------------
 guix/scripts/import/opam.scm |   8 +-
 tests/opam.scm               |  68 ++++++++-------
 4 files changed, 160 insertions(+), 104 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 78c1c09858..2d36561186 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -94,6 +94,7 @@ Copyright @copyright{} 2021 Xinglu Chen@*
 Copyright @copyright{} 2021 Raghav Gururajan@*
 Copyright @copyright{} 2021 Domagoj Stolfa@*
 Copyright @copyright{} 2021 Hui Lu@*
+Copyright @copyright{} 2021 Alice Brenon@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -11612,14 +11613,31 @@ Traverse the dependency graph of the given upstream package recursively
 and generate package expressions for all those packages that are not yet
 in Guix.
 @item --repo
-Select the given repository (a repository name).  Possible values include:
+
+By default, packages are searched in the official OPAM repository.  This
+option, which can be used more than once, lets you add other
+repositories which will be searched for packages.  It accepts as valid
+arguments:
+
 @itemize
-@item @code{opam}, the default opam repository,
-@item @code{coq} or @code{coq-released}, the stable repository for coq packages,
-@item @code{coq-core-dev}, the repository that contains development versions of coq,
-@item @code{coq-extra-dev}, the repository that contains development versions
-      of coq packages.
+@item the name of a known repository - can be one of @code{opam},
+      @code{coq} (equivalent to @code{coq-released}),
+	  @code{coq-core-dev}, @code{coq-extra-dev} or @code{grew}.
+@item the URL of a repository as expected by the @code{opam repository
+      add} command (for instance, the URL equivalent of the above
+	  @code{opam} name would be @uref{https://opam.ocaml.org}).
+@item the path to a local copy of a repository (a directory containing a
+      @file{packages/} sub-directory).
 @end itemize
+
+Repositories must be passed to this option by order of preference and do
+not replace the default @code{opam} which is always failed-back to.
+
+Also, please note that versions are not compared accross repositories.
+The first repository (from left to right) that has at least one version
+of a given package will prevail over any others and the version imported
+will be the latest one found @emph{in this repository only}.
+
 @end table
 
 @item go
diff --git a/guix/import/opam.scm b/guix/import/opam.scm
index a35b01d277..fe13d29f03 100644
--- a/guix/import/opam.scm
+++ b/guix/import/opam.scm
@@ -2,6 +2,7 @@
 ;;; Copyright © 2018 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2020 Martin Becze <mjbecze@riseup.net>
 ;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz>
+;;; Copyright © 2021 Alice Brenon <alice.brenon@ens-lyon.fr>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -22,21 +23,24 @@
   #:use-module (ice-9 ftw)
   #:use-module (ice-9 match)
   #:use-module (ice-9 peg)
+  #:use-module ((ice-9 popen) #:select (open-pipe*))
   #:use-module (ice-9 receive)
-  #:use-module ((ice-9 rdelim) #:select (read-line))
   #:use-module (ice-9 textual-ports)
   #:use-module (ice-9 vlist)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-2)
-  #:use-module (web uri)
+  #:use-module ((srfi srfi-26) #:select (cut))
+  #:use-module ((web uri) #:select (string->uri uri->string))
+  #:use-module ((guix build utils) #:select (dump-port find-files mkdir-p))
   #:use-module (guix build-system)
   #:use-module (guix build-system ocaml)
   #:use-module (guix http-client)
-  #:use-module (guix git)
   #:use-module (guix ui)
   #:use-module (guix packages)
   #:use-module (guix upstream)
-  #:use-module (guix utils)
+  #:use-module ((guix utils) #:select (cache-directory
+                                       version>?
+                                       call-with-temporary-output-file))
   #:use-module (guix import utils)
   #:use-module ((guix licenses) #:prefix license:)
   #:export (opam->guix-package
@@ -121,51 +125,83 @@
 (define-peg-pattern condition-string all (and QUOTE (* STRCHR) QUOTE))
 (define-peg-pattern condition-var all (+ (or (range #\a #\z) "-" ":")))
 
-(define* (get-opam-repository #:optional repo)
+(define (opam-cache-directory path)
+  (string-append (cache-directory) "/opam/" path))
+
+(define known-repositories
+  '((opam . "https://opam.ocaml.org")
+    (coq . "https://coq.inria.fr/opam/released")
+    (coq-released . "https://coq.inria.fr/opam/released")
+    (coq-core-dev . "https://coq.inria.fr/opam/core-dev")
+    (coq-extra-dev . "https://coq.inria.fr/opam/extra-dev")
+    (grew . "http://opam.grew.fr")))
+
+(define (get-uri repo-root)
+  (let ((archive-file (string-append repo-root "/index.tar.gz")))
+    (or (string->uri archive-file)
+        (begin
+          (warning (G_ "'~a' is not a valid URI~%") archive-file)
+          'bad-repo))))
+
+(define (repo-type repo)
+  (match (assoc-ref known-repositories (string->symbol repo))
+    (#f (if (file-exists? repo)
+            `(local ,repo)
+            `(remote ,(get-uri repo))))
+    (url `(remote ,(get-uri url)))))
+
+(define (update-repository input)
+  "Make sure the cache for opam repository INPUT is up-to-date"
+  (let* ((output (opam-cache-directory (basename (port-filename input))))
+         (cached-date (if (file-exists? output)
+                          (stat:mtime (stat output))
+                          (begin (mkdir-p output) 0))))
+    (when (> (stat:mtime (stat input)) cached-date)
+      (call-with-port
+       (open-pipe* OPEN_WRITE "tar" "xz" "-C" output "-f" "-")
+       (cut dump-port input <>)))
+    output))
+
+(define* (get-opam-repository #:optional (repo "opam"))
   "Update or fetch the latest version of the opam repository and return the
 path to the repository."
-  (let ((url (cond
-               ((or (not repo) (equal? repo 'opam))
-                "https://github.com/ocaml/opam-repository")
-               ((string-prefix? "coq-" (symbol->string repo))
-                "https://github.com/coq/opam-coq-archive")
-               ((equal? repo 'coq) "https://github.com/coq/opam-coq-archive")
-               (else (throw 'unknown-repository repo)))))
-    (receive (location commit _)
-      (update-cached-checkout url)
-      (cond
-        ((or (not repo) (equal? repo 'opam))
-         location)
-        ((equal? repo 'coq)
-         (string-append location "/released"))
-        ((string-prefix? "coq-" (symbol->string repo))
-         (string-append location "/" (substring (symbol->string repo) 4)))
-        (else location)))))
+  (match (repo-type repo)
+    (('local p) p)
+    (('remote 'bad-repo) #f) ; to weed it out during filter-map in opam-fetch
+    (('remote r) (call-with-port (http-fetch/cached r) update-repository))))
 
 ;; Prevent Guile 3 from inlining this procedure so we can mock it in tests.
 (set! get-opam-repository get-opam-repository)
 
-(define (latest-version versions)
-  "Find the most recent version from a list of versions."
-  (fold (lambda (a b) (if (version>? a b) a b)) (car versions) versions))
+(define (get-version-and-file path)
+  "Analyse a candidate path and return an list containing information for proper
+  version comparison as well as the source path for metadata."
+  (and-let* ((metadata-file (string-append path "/opam"))
+             (filename (basename path))
+             (version (string-join (cdr (string-split filename #\.)) ".")))
+    (and (file-exists? metadata-file)
+         (eq? 'regular (stat:type (stat metadata-file)))
+         (if (string-prefix? "v" version)
+             `(V ,(substring version 1) ,metadata-file)
+             `(digits ,version ,metadata-file)))))
+
+(define (keep-max-version a b)
+  "Version comparison on the lists returned by the previous function taking the
+  janestreet re-versioning into account (v-prefixed come first)."
+  (match (cons a b)
+    ((('V va _) . ('V vb _)) (if (version>? va vb) a b))
+    ((('V _ _) . _) a)
+    ((_ . ('V _ _)) b)
+    ((('digits va _) . ('digits vb _)) (if (version>? va vb) a b))))
 
 (define (find-latest-version package repository)
   "Get the latest version of a package as described in the given repository."
-  (let* ((dir (string-append repository "/packages/" package))
-         (versions (scandir dir (lambda (name) (not (string-prefix? "." name))))))
-    (if versions
-      (let ((versions (map
-                        (lambda (dir)
-                          (string-join (cdr (string-split dir #\.)) "."))
-                        versions)))
-        ;; Workaround for janestreet re-versionning
-        (let ((v-versions (filter (lambda (version) (string-prefix? "v" version)) versions)))
-          (if (null? v-versions)
-            (latest-version versions)
-            (string-append "v" (latest-version (map (lambda (version) (substring version 1)) v-versions))))))
-      (begin
-        (format #t (G_ "Package not found in opam repository: ~a~%") package)
-        #f))))
+  (let ((packages (string-append repository "/packages"))
+        (filter (make-regexp (string-append "^" package "\\."))))
+    (reduce keep-max-version #f
+            (filter-map
+             get-version-and-file
+             (find-files packages filter #:directories? #t)))))
 
 (define (get-metadata opam-file)
   (with-input-from-file opam-file
@@ -266,28 +302,30 @@ path to the repository."
 
 (define (depends->native-inputs depends)
   (filter (lambda (name) (not (equal? "" name)))
-    (map dependency->native-input depends)))
+          (map dependency->native-input depends)))
 
 (define (dependency-list->inputs lst)
   (map
-    (lambda (dependency)
-      (list dependency (list 'unquote (string->symbol dependency))))
-    (ocaml-names->guix-names lst)))
-
-(define* (opam-fetch name #:optional (repository (get-opam-repository)))
-  (and-let* ((repository repository)
-             (version (find-latest-version name repository))
-             (file (string-append repository "/packages/" name "/" name "." version "/opam")))
-    `(("metadata" ,@(get-metadata file))
-      ("version" . ,(if (string-prefix? "v" version)
-                        (substring version 1)
-                        version)))))
-
-(define* (opam->guix-package name #:key (repo 'opam) version)
-  "Import OPAM package NAME from REPOSITORY (a directory name) or, if
-REPOSITORY is #f, from the official OPAM repository.  Return a 'package' sexp
+   (lambda (dependency)
+     (list dependency (list 'unquote (string->symbol dependency))))
+   (ocaml-names->guix-names lst)))
+
+(define* (opam-fetch name #:optional (repositories-specs '("opam")))
+  (or (fold (lambda (repository others)
+              (match (find-latest-version name repository)
+                ((_ version file) `(("metadata" ,@(get-metadata file))
+                                    ("version" . ,version)))
+                (_ others)))
+            #f
+            (filter-map get-opam-repository repositories-specs))
+      (leave (G_ "package '~a' not found~%") name)))
+
+(define* (opam->guix-package name #:key (repo '()) version)
+  "Import OPAM package NAME from REPOSITORIES (a list of names, URLs or local
+paths, always including OPAM's official repository).  Return a 'package' sexp
 or #f on failure."
-  (and-let* ((opam-file (opam-fetch name (get-opam-repository repo)))
+  (and-let* ((with-opam (if (member "opam" repo) repo (cons "opam" repo)))
+             (opam-file (opam-fetch name with-opam))
              (version (assoc-ref opam-file "version"))
              (opam-content (assoc-ref opam-file "metadata"))
              (url-dict (metadata-ref opam-content "url"))
@@ -312,9 +350,7 @@ or #f on failure."
                    (values
                     `(package
                        (name ,(ocaml-name->guix-name name))
-                       (version ,(if (string-prefix? "v" version)
-                                   (substring version 1)
-                                   version))
+                       (version ,version)
                        (source
                          (origin
                            (method url-fetch)
diff --git a/guix/scripts/import/opam.scm b/guix/scripts/import/opam.scm
index 64164e7cc4..834ac34cb0 100644
--- a/guix/scripts/import/opam.scm
+++ b/guix/scripts/import/opam.scm
@@ -1,6 +1,7 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2018 Julien Lepiller <julien@lepiller.eu>
 ;;; Copyright © 2021 Sarah Morgensen <iskarian@mgsn.dev>
+;;; Copyright © 2021 Alice Brenon <alice.brenon@ens-lyon.fr>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -46,7 +47,8 @@ Import and convert the opam package for PACKAGE-NAME.\n"))
   (display (G_ "
   -r, --recursive        import packages recursively"))
   (display (G_ "
-      --repo             import packages from this opam repository"))
+      --repo             import packages from this opam repository (name, URL or local path)
+                         can be used more than once"))
   (display (G_ "
   -V, --version          display version information and exit"))
   (newline)
@@ -81,7 +83,9 @@ Import and convert the opam package for PACKAGE-NAME.\n"))
                         #:build-options? #f))
 
   (let* ((opts (parse-options))
-         (repo (and=> (assoc-ref opts 'repo) string->symbol))
+         (repo (filter-map (match-lambda
+                             (('repo . name) name)
+                             (_ #f)) opts))
          (args (filter-map (match-lambda
                             (('argument . value)
                              value)
diff --git a/tests/opam.scm b/tests/opam.scm
index f1e3b70cb0..1536b74339 100644
--- a/tests/opam.scm
+++ b/tests/opam.scm
@@ -82,41 +82,39 @@ url {
                 (set! test-source-hash
                   (call-with-input-file file-name port-sha256))))
              (_ (error "Unexpected URL: " url)))))
-        (mock ((guix import opam) get-opam-repository
-               (const test-repo))
-              (let ((my-package (string-append test-repo
-                                               "/packages/foo/foo.1.0.0")))
-                (mkdir-p my-package)
-                (with-output-to-file (string-append my-package "/opam")
-                  (lambda _
-                    (format #t "~a" test-opam-file))))
-              (match (opam->guix-package "foo" #:repo test-repo)
-                (('package
-                   ('name "ocaml-foo")
-                   ('version "1.0.0")
-                   ('source ('origin
-                              ('method 'url-fetch)
-                              ('uri "https://example.org/foo-1.0.0.tar.gz")
-                              ('sha256
-                               ('base32
-                                (? string? hash)))))
-                   ('build-system 'ocaml-build-system)
-                   ('propagated-inputs
-                    ('quasiquote
-                     (("ocaml-zarith" ('unquote 'ocaml-zarith)))))
-                   ('native-inputs
-                    ('quasiquote
-                     (("ocaml-alcotest" ('unquote 'ocaml-alcotest))
-                      ("ocamlbuild" ('unquote 'ocamlbuild)))))
-                   ('home-page "https://example.org/")
-                   ('synopsis "Some example package")
-                   ('description "This package is just an example.")
-                   ('license 'license:bsd-3))
-                 (string=? (bytevector->nix-base32-string
-                            test-source-hash)
-                           hash))
-                (x
-                 (pk 'fail x #f))))))
+        (let ((my-package (string-append test-repo
+                                         "/packages/foo/foo.1.0.0")))
+          (mkdir-p my-package)
+          (with-output-to-file (string-append my-package "/opam")
+            (lambda _
+              (format #t "~a" test-opam-file))))
+        (match (opam->guix-package "foo" #:repo (list test-repo))
+          (('package
+             ('name "ocaml-foo")
+             ('version "1.0.0")
+             ('source ('origin
+                        ('method 'url-fetch)
+                        ('uri "https://example.org/foo-1.0.0.tar.gz")
+                        ('sha256
+                         ('base32
+                          (? string? hash)))))
+             ('build-system 'ocaml-build-system)
+             ('propagated-inputs
+              ('quasiquote
+               (("ocaml-zarith" ('unquote 'ocaml-zarith)))))
+             ('native-inputs
+              ('quasiquote
+               (("ocaml-alcotest" ('unquote 'ocaml-alcotest))
+                ("ocamlbuild" ('unquote 'ocamlbuild)))))
+             ('home-page "https://example.org/")
+             ('synopsis "Some example package")
+             ('description "This package is just an example.")
+             ('license 'license:bsd-3))
+           (string=? (bytevector->nix-base32-string
+                      test-source-hash)
+                     hash))
+          (x
+           (pk 'fail x #f)))))
 
 ;; Test the opam file parser
 ;; We fold over some test cases. Each case is a pair of the string to parse and the
-- 
2.32.0