diff --git a/.github/workflows/python-package.yml b/.github/workflows/python-package.yml
index 08ca30f7..a53ffdb7 100644
--- a/.github/workflows/python-package.yml
+++ b/.github/workflows/python-package.yml
@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12"]
 
     steps:
     - uses: actions/checkout@v3
diff --git a/docs/source/fundamentals/substitutions.rst b/docs/source/fundamentals/substitutions.rst
index efed7a64..e21e054b 100644
--- a/docs/source/fundamentals/substitutions.rst
+++ b/docs/source/fundamentals/substitutions.rst
@@ -169,6 +169,15 @@ As we saw above, a parameter value starting with ``=`` invokes the formula parse
    
   * ``STRIPEXT(`` *path* ``)`` returns the path minus the extension. 
 
+  * ``IS_NUM(arg)`` true if the argument is a numeric type.
+
+  * ``IS_STR(arg)`` true if the argument is a string type.
+
+  * ``VALID(arg)`` true if the argument is valid, and evaluates to non-zero. This is a useful pattern when dealing 
+  with parameters of a mixed type (that can be e.g. strings or numbers). For example, ``recipe.a > 0`` would throw an 
+  error is ``a`` is a string, but ``VALID(recipe.a > 0)`` would return False in this case.
+  
+
 As should be evident from the list above, certain functions expect arguments of a particular type (for example, the pathname manipulation functions expect strings). 
 
 Note that function arguments are treated as fully-fledged expressions of their own (with the exception of the first argument of ``IFSET()``, which must be a namespace lookup by definition.) In particular, {}-substitutions are applied to string arguments. For example, the following can be a legit (and useful) invocation::
diff --git a/pyproject.toml b/pyproject.toml
index 0be688d9..e526084c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -14,7 +14,8 @@ packages = [
 ]
 
 [tool.poetry.dependencies]
-python = "^3.8"
+python = "^3.9"
+numpy = "^1.20"
 munch = "^2.5.0"
 omegaconf = "^2.1"
 importlib_metadata = { version = "4.13.0", python = "3.7" }
diff --git a/scabha/evaluator.py b/scabha/evaluator.py
index 06f873c2..31b4be0d 100644
--- a/scabha/evaluator.py
+++ b/scabha/evaluator.py
@@ -3,6 +3,7 @@
 import os.path
 import fnmatch
 import pyparsing
+import numpy as np
 pyparsing.ParserElement.enable_packrat()
 from pyparsing import *
 from pyparsing import common
@@ -156,12 +157,33 @@ def RANGE(self, evaluator, args):
         def make_range(*x):
             return list(range(*x))
         return self.evaluate_generic_callable(evaluator, "RANGE", make_range, args, min_args=1, max_args=3)
+    
+    def VALID(self, evaluator, args):
+        if len(args) != 1:
+            raise FormulaError(f"{'.'.join(evaluator.location)}: VALID() expects one argument, got {len(args)}")
+        try:
+            result = evaluator._evaluate_result(args[0], allow_unset=True)
+        except (TypeError, ValueError) as exc:
+            return False
+        if type(result) is UNSET:
+            return False
+        return result
 
     def MIN(self, evaluator, args):
         return self.evaluate_generic_callable(evaluator, "MIN", min, args, min_args=1)
 
     def MAX(self, evaluator, args):
         return self.evaluate_generic_callable(evaluator, "MAX", max, args, min_args=1)
+    
+    def IS_STR(self, evaluator, args):
+        def is_str(x):
+            return type(x) is str
+        return self.evaluate_generic_callable(evaluator, "IS_STR", is_str, args, min_args=1, max_args=1)
+    
+    def IS_NUM(self, evaluator, args):
+        def is_num(x):
+            return np.isscalar(x) and (np.isreal(x) or np.isscalar(x)) 
+        return self.evaluate_generic_callable(evaluator, "IS_NUM", is_num, args, min_args=1, max_args=1)
 
     def IF(self, evaluator, args):
         if len(args) < 3 or len(args) > 4:
@@ -300,7 +322,7 @@ def construct_parser():
     
     # functions
     functions = reduce(operator.or_, map(Keyword, ["IF", "IFSET", "GLOB", "EXISTS", "LIST", 
-        "BASENAME", "DIRNAME", "EXTENSION", "STRIPEXT", "MIN", "MAX", "RANGE", "NOSUBST", "SORT", "RSORT"]))
+        "BASENAME", "DIRNAME", "EXTENSION", "STRIPEXT", "MIN", "MAX", "IS_STR", "IS_NUM", "VALID", "RANGE", "NOSUBST", "SORT", "RSORT"]))
     # these functions take one argument, which could also be a sequence
     anyseq_functions = reduce(operator.or_, map(Keyword, ["GLOB", "EXISTS"]))