feat(tazjin/nisp): Add initial source-to-source transform

... this isn't finished yet, in particular it lacks:

* better support for attribute sets
* support for defining functions that take attribute sets

Change-Id: Ia897fccd9d2b674b6ed12907ae297bfdcc86db48
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2237
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/nisp/transform.el

@@ -0,0 +1,137 @@
+;; Nix as a Lisp
+
+(require 'cl-lib)
+(require 'json)
+(require 's)
+(require 'dash)
+
+(defun nisp/expr (form)
+  "Entrypoint for Nisp->Nix transformation. Will translate FORM
+into Nix code, if it is a valid Nisp expression.
+
+To make code generation slightly easier, each
+expression (including literals) is wrapped in an extra pair of
+parens."
+  (concat
+   "("
+   (pcase form
+     ;; Special keywords
+     ('() "null")
+     (`(let . ,rest) (nisp/let form))
+     (`(fn . ,rest) (nisp/fn form))
+     (`(if ,cond ,then ,else) (nisp/if cond then else))
+
+     ;; Nix operators & builtins that need special handling
+     (`(or  ,lhs ,rhs) (nisp/infix "||" lhs rhs))
+     (`(and ,lhs ,rhs) (nisp/infix "&&" lhs rhs))
+     (`(> ,lhs ,rhs) (nisp/infix ">" lhs rhs))
+     (`(< ,lhs ,rhs) (nisp/infix "<" lhs rhs))
+     (`(>= ,lhs ,rhs) (nisp/infix ">=" lhs rhs))
+     (`(<= ,lhs ,rhs) (nisp/infix "<=" lhs rhs))
+     (`(+ ,lhs ,rhs) (nisp/infix "+" lhs rhs))
+     (`(- ,lhs ,rhs) (nisp/infix "-" lhs rhs))
+     (`(* ,lhs ,rhs) (nisp/infix "*" lhs rhs))
+     (`(/ ,lhs ,rhs) (nisp/infix "/" lhs rhs))
+     (`(-> ,lhs ,rhs) (nisp/infix "->" lhs rhs))
+     (`(? ,lhs ,rhs) (nisp/infix "?" lhs rhs))
+     (`(// ,lhs ,rhs) (nisp/infix "//" lhs rhs))
+     (`(++ ,lhs ,rhs) (nisp/infix "++" lhs rhs))
+     (`(== ,lhs ,rhs) (nisp/infix "==" lhs rhs))
+     (`(!= ,lhs ,rhs) (nisp/infix "!=" lhs rhs))
+     (`(! ,term) (concat "!" (nisp/expr term)))
+     (`(- ,term) (concat "-" (nisp/expr term)))
+
+     ;; Attribute sets
+     (`(attrs . ,rest) (nisp/attribute-set form))
+
+     ;; Function calls
+     ((and `(,func . ,args)
+           (guard (symbolp func)))
+      (nisp/funcall func args))
+
+     ;; Primitives
+     ((pred stringp) (json-encode-string form))
+     ((pred numberp) (json-encode-number form))
+     ((pred keywordp) (substring (symbol-name form) 1))
+     ((pred symbolp) (symbol-name form))
+
+     ;; Lists
+     ((pred arrayp) (nisp/list form))
+
+     (other (error "Encountered unhandled form: %s" other)))
+   ")"))
+
+(defun nisp/infix (op lhs rhs)
+  (concat (nisp/expr lhs) " " op " " (nisp/expr rhs)))
+
+(defun nisp/funcall (func args)
+  (concat (symbol-name func) " " (s-join " " (-map #'nisp/expr args))))
+
+(defun nisp/let (form)
+  (pcase form
+    (`(let . (,bindings . (,body . ()))) (concat "let "
+                                                 (nisp/let bindings)
+                                                 (nisp/expr body)))
+    (`((:inherit . ,inherits) . ,rest) (concat (nisp/inherit (car form))
+                                               " "
+                                               (nisp/let rest)))
+    (`((,name . (,value . ())) .,rest) (concat (symbol-name name) " = "
+                                               (nisp/expr value) "; "
+                                               (nisp/let rest)))
+    ('() "in ")
+    (other (error "malformed form '%s' in let expression" other))))
+
+(defun nisp/inherit (form)
+  (pcase form
+    (`(:inherit . ,rest) (concat "inherit " (nisp/inherit rest)))
+    (`((,source) . ,rest) (concat "(" (symbol-name source) ") " (nisp/inherit rest)))
+    (`(,item . ,rest) (concat (symbol-name item) " " (nisp/inherit rest)))
+    ('() ";")))
+
+(defun nisp/if (cond then else)
+  (concat "if " (nisp/expr cond)
+          " then " (nisp/expr then)
+          " else " (nisp/expr else)))
+
+(defun nisp/list (form)
+  (cl-check-type form array)
+  (concat "[ "
+          (mapconcat #'nisp/expr form " ")
+          "]"))
+
+
+(defun nisp/attribute-set (form)
+  "Attribute sets have spooky special handling because they are
+not supported by the reader."
+  (pcase form
+    (`(attrs . ,rest) (concat "{ " (nisp/attribute-set rest)))
+    ((and `(,name . (,value . ,rest))
+          (guard (keywordp name)))
+     (concat (substring (symbol-name name) 1) " = "
+             (nisp/expr value) "; "
+             (nisp/attribute-set rest)))
+    ('() "}")))
+
+(defun nisp/fn (form)
+  (pcase form
+    (`(fn ,args ,body) (concat
+                              (cl-loop for arg in args
+                                       concat (format "%s: " arg))
+                              (nisp/expr body)))))
+
+;; The following functions are not part of the transform.
+
+(defun nisp/eval (form)
+  (interactive "sExpression: ")
+  (when (stringp form)
+    (setq form (read form)))
+
+  (message
+   ;; TODO(tazjin): Construct argv manually to avoid quoting issues.
+   (s-chomp
+    (shell-command-to-string
+     (concat "nix-instantiate --eval -E '" (nisp/expr form) "'")))))
+
+(defun nisp/eval-last-sexp ()
+  (interactive)
+  (nisp/eval (edebug-last-sexp)))