Add isPath primop

this is added for completeness' sake since all the other possible
`builtins.typeOf` results have a corresponding `builtins.is<Type>`

3 files changed, 17 insertions, 0 deletions

diff --git a/doc/manual/expressions/builtins.xml b/doc/manual/expressions/builtins.xml
index cefc8b78eebe..0fb5261b384c 100644
--- a/doc/manual/expressions/builtins.xml
+++ b/doc/manual/expressions/builtins.xml
@@ -861,6 +861,14 @@ x: x + 456</programlisting>
 
   </varlistentry>
 
+  <varlistentry><term><function>builtins.isPath</function>
+  <replaceable>e</replaceable></term>
+
+    <listitem><para>Return <literal>true</literal> if
+    <replaceable>e</replaceable> evaluates to a path, and
+    <literal>false</literal> otherwise.</para></listitem>
+
+  </varlistentry>
 
   <varlistentry xml:id='builtin-isNull'>
     <term><function>isNull</function>
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 6b0c55e72cd9..39073725e9c4 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -315,6 +315,12 @@ static void prim_isBool(EvalState & state, const Pos & pos, Value * * args, Valu
     mkBool(v, args[0]->type == tBool);
 }
 
+/* Determine whether the argument is a path. */
+static void prim_isPath(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceValue(*args[0]);
+    mkBool(v, args[0]->type == tPath);
+}
 
 struct CompareValues
 {
@@ -2169,6 +2175,7 @@ void EvalState::createBaseEnv()
     addPrimOp("__isInt", 1, prim_isInt);
     addPrimOp("__isFloat", 1, prim_isFloat);
     addPrimOp("__isBool", 1, prim_isBool);
+    addPrimOp("__isPath", 1, prim_isPath);
     addPrimOp("__genericClosure", 1, prim_genericClosure);
     addPrimOp("abort", 1, prim_abort);
     addPrimOp("__addErrorContext", 2, prim_addErrorContext);
diff --git a/tests/lang/eval-okay-types.nix b/tests/lang/eval-okay-types.nix
index a34775f5e602..9b58be5d1dd4 100644
--- a/tests/lang/eval-okay-types.nix
+++ b/tests/lang/eval-okay-types.nix
@@ -20,6 +20,8 @@ with builtins;
   (isFloat (1 - 2.0))
   (isBool (true && false))
   (isBool null)
+  (isPath /nix/store)
+  (isPath ./.)
   (isAttrs { x = 123; })
   (isAttrs null)
   (typeOf (3 * 4))