added doc + runnableExamples for lambdaEval

lib/pure/lambda.nim

@@ -1,9 +1,22 @@
 import macros
 
 macro lambdaEval*(lambda: untyped, arg:typed):untyped=
+  ## allows using zero-cost lambda expressions ``a=>b`` in templates. Type
+  ## inference is done at evaluation site, so user doesn't need to specify
+  ## types in the lambda expression.
+  runnableExamples:
+    template testCallFun[T](fun: untyped, a:T): auto =
+      lambdaEval(fun, lambdaEval(fun, a))
+    doAssert testCallFun(x => x * 3, 2) == (2 * 3) * 3
+
   if $lambda[0] != "=>":
     error("Expected `=>` got " & $lambda[0])
   var ret = newStmtList()
   ret.add newLetStmt(lambda[1], arg)
   ret.add lambda[2]
   result = newBlockStmt(ret)
+
+when isMainModule:
+  block lambdaEvalTest:
+    # PENDING https://github.com/nim-lang/Nim/issues/7280
+    discard lambdaEval(a => a, 0)