kernels: Define <kernel-specs>.

* jupyter/kernels.scm (<kernel-specs>): New record type.
(kernel-arguments): Use 'json->kernel-specs' and 'kernel-specs-arguments'.

jupyter/kernels.scm

 ;;; Guix-kernel -- Guix kernel for Jupyter
-;;; Copyright (C) 2018 Pierre-Antoine Rouby <pierre-antoine.rouby@inria.fr>
+;;; Copyright (C) 2018, 2019 Pierre-Antoine Rouby <pierre-antoine.rouby@inria.fr>
 ;;; Copyright (C) 2018, 2019 Ludovic Courtès <ludovic.courtes@inria.fr>
 ;;;
 ;;; This program is free software: you can redistribute it and/or modify
@@ -17,6 +17,7 @@
 
 (define-module (jupyter kernels)
   #:use-module (jupyter messages)
+  #:use-module (jupyter json)
   #:use-module (guix build utils)
   #:use-module (ice-9 rdelim)
   #:use-module (ice-9 match)
@@ -29,6 +30,12 @@
   #:export (find-kernel-by-name
             run-kernel
 
+            kernel-specs?
+            kernel-specs-arguments
+            kernel-specs-display-name
+            kernel-specs-language
+            json->kernel-specs
+
             kernel
             kernel?
             kernel-name
@@ -78,6 +85,16 @@
 
 (define-syntax kernel-iopub (identifier-syntax kernel-iosub))
 
+;; Kernel metadata taken from a 'kernel.json' file.
+;; <https://jupyter-client.readthedocs.io/en/latest/kernels.html#kernel-specs>
+(define-json-mapping <kernel-specs>
+  kernel-specs kernel-specs?
+  json->kernel-specs
+  (arguments      kernel-specs-arguments "argv"
+                  vector->list)
+  (display-name   kernel-specs-display-name "display_name")
+  (language       kernel-specs-language))
+
 (define* (kernel name pid #:key key control shell
                  standard-input heartbeat iosub)
   (%kernel name pid key
@@ -166,11 +183,11 @@ sockets."
 (define (kernel-arguments kernel-file connection-file)
   "Return the argument list read from KERNEL-FILE, substituting references to
 'connection-file' with CONNECTION-FILE."
-  (let ((alist (call-with-input-file kernel-file json->scm)))
+  (let ((specs (call-with-input-file kernel-file json->kernel-specs)))
     (map (match-lambda
            ("{connection_file}" connection-file)
            (x x))
-         (vector->list (assoc-ref alist "argv")))))
+         (kernel-specs-arguments specs))))
 
 (define search-path->list
   (let ((not-colon (char-set-complement (char-set #\:))))