(WIP) Add a contracts.yants compatibility attribute set

https://code.tvl.fyi/about/nix/yants

default.nix

@@ -68,7 +68,7 @@ is = contract { message = s: "Value should be of type `${s.type}':"; };
 # TODO I'm not sure if this would be really handy:
 #     FunctionArgs = args: f: args == builtins.functionArgs f;
 # ... because I would prefer something like:
-fn = Args: f: x: f (is (def Args) x);
+fn = arg: f: x: f (is (def arg) x);
 
 /* Force the concrete evaluation of a contract or any datatype */
 strict = e: builtins.deepSeq e e;
@@ -135,6 +135,10 @@ match   = regex: # FIXME check if `regex` is of `Regex` type!
 # From https://www.rfc-editor.org/rfc/rfc3986#page-50
 Url     = match ''^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?'';
 
+Drv = declare { name = "Derivation"; } { type = "derivation"; };
+Maybe = declare { name = "Maybe"; } (x: enum [ Null x ]);
+Unit = declare { name = "{}"; } { }; # TODO: This just behaves like Set type ...
+
 /* *** Some type ideas for the future *** */
 Hash    = TODO; # FIXME SRI hashes used by Nix :)
 Regex   = TODO; # It would be nice to check if a RegEx is valid ...
@@ -146,7 +150,44 @@ SemVer  = TODO; # ... or if a software version is in SemVer format!
 # This is internal mixture that helps to have declared types available here:
 prelude = (builtins.mapAttrs (n: _: declare { name = n; } types.${n}) types);
 
+# Compatibility with https://code.tvl.fyi/about/nix/yants :)
+# blob: cb9fc08287fb1f81159d996947c0aaf1cc1672be
+
+yants = with prelude;
+let opt = x: if isString arg then (declare { name = x; }) else (def arg); in
+rec {
+  any = Any;
+  attrs = setOf;
+  bool = Bool;
+
+  # FIXME: This buggy and not yet understand how to write it correctly ...
+  # FIXME: Should declare it as { name = "λ :: A -> B -> C" }?
+  defun = args: f: a: let x = builtins.head args; xs = builtins.tail args; in
+            fn x (if builtins.length args > 1 then (defun xs f) else (f a));
+
+  drv = Drv;
+  either = t1: t2: enum [ t1 t2 ];
+  eitherN = enum;
+
+  # FIXME: Does not support pattern matching as .match "foo" { foo = ...; }
+  enum = opt enum;
+
+  float = Float;
+  function = enum [ Lambda Functor ];
+  int = Int;
+  list = listOf;
+  null = Null;
+  option = Maybe;
+  path = Path;
+  restrict = name: pred: t: x: contract {name = name; } (pred (def t x));
+  string = String;
+  struct = opt;
+  sum = opt both;
+  type = Type;
+  unit = Unit;
+};
+
 in prelude // {
   # Explicitly choose what would be our library interface (and eventually not)
-  inherit declare def default contract is fn strict;
+  inherit declare def default contract is fn strict yants;
 }