diff --git a/ognibuild/__main__.py b/ognibuild/__main__.py
index af14c31..3a40ef1 100644
--- a/ognibuild/__main__.py
+++ b/ognibuild/__main__.py
@@ -35,8 +35,8 @@ def get_necessary_declared_requirements(resolver, requirements, stages):
     return missing
 
 
-def install_necessary_declared_requirements(resolver, buildsystem, stages):
-    missing = []
+def install_necessary_declared_requirements(session, resolver, buildsystem, stages):
+    relevant = []
     try:
         declared_reqs = list(buildsystem.get_declared_dependencies())
     except NotImplementedError:
@@ -44,10 +44,18 @@ def install_necessary_declared_requirements(resolver, buildsystem, stages):
             "Unable to determine declared dependencies from %s", buildsystem
         )
     else:
-        missing.extend(
+        relevant.extend(
             get_necessary_declared_requirements(resolver, declared_reqs, stages)
         )
-    resolver.install(missing)
+    missing = []
+    for req in relevant:
+        try:
+            if not req.met(session):
+                missing.append(req)
+        except NotImplementedError:
+            missing.append(req)
+    if missing:
+        resolver.install(missing)
 
 
 # Types of dependencies:
@@ -141,7 +149,7 @@ def main():  # noqa: C901
                 if stages:
                     logging.info("Checking that declared requirements are present")
                     for bs in bss:
-                        install_necessary_declared_requirements(resolver, bs, stages)
+                        install_necessary_declared_requirements(session, resolver, bs, stages)
             fixers = determine_fixers(session, resolver)
             if args.subcommand == "dist":
                 from .dist import run_dist
diff --git a/ognibuild/buildlog.py b/ognibuild/buildlog.py
index 8a8c1e3..33d51ed 100644
--- a/ognibuild/buildlog.py
+++ b/ognibuild/buildlog.py
@@ -120,7 +120,7 @@ def problem_to_upstream_requirement(problem):  # noqa: C901
     elif isinstance(problem, MissingJavaClass):
         return JavaClassRequirement(problem.classname)
     elif isinstance(problem, MissingHaskellDependencies):
-        return [HaskellPackageRequirement(dep) for dep in problem.deps]
+        return [HaskellPackageRequirement.from_string(dep) for dep in problem.deps]
     elif isinstance(problem, MissingMavenArtifacts):
         return [MavenArtifactRequirement(artifact) for artifact in problem.artifacts]
     elif isinstance(problem, MissingCSharpCompiler):
diff --git a/ognibuild/buildsystem.py b/ognibuild/buildsystem.py
index a951ee7..5291b4f 100644
--- a/ognibuild/buildsystem.py
+++ b/ognibuild/buildsystem.py
@@ -633,6 +633,9 @@ class Cabal(BuildSystem):
     def test(self, session, resolver, fixers):
         self._run(session, ["test"], fixers)
 
+    def dist(self, session, resolver, fixers, quiet=False):
+        self._run(session, ["sdist"], fixers)
+
 
 def detect_buildsystems(path, trust_package=False):  # noqa: C901
     """Detect build systems."""
diff --git a/ognibuild/requirements.py b/ognibuild/requirements.py
index 560ad29..2f3673f 100644
--- a/ognibuild/requirements.py
+++ b/ognibuild/requirements.py
@@ -17,6 +17,7 @@
 # Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 
 import posixpath
+import subprocess
 from typing import Optional, List, Tuple
 
 from . import Requirement
@@ -55,6 +56,26 @@ class PythonPackageRequirement(Requirement):
         req = Requirement.parse(text)
         return cls(package=req.name, specs=req.specs)
 
+    def met(self, session):
+        if self.python_version == "cpython3":
+            cmd = "python3"
+        elif self.python_version == "cpython2":
+            cmd = "python2"
+        elif self.python_version == "pypy":
+            cmd = "pypy"
+        elif self.python_version == "pypy3":
+            cmd = "pypy3"
+        elif self.python_version is None:
+            cmd = "python3"
+        else:
+            raise NotImplementedError
+        text = self.package + ','.join([''.join(spec) for spec in self.specs])
+        p = session.Popen(
+            [cmd, "-c", "import pkg_resources; pkg_resources.require(%r)" % text],
+            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        p.communicate()
+        return p.returncode == 0
+
 
 class BinaryRequirement(Requirement):
 
@@ -64,6 +85,13 @@ class BinaryRequirement(Requirement):
         super(BinaryRequirement, self).__init__("binary")
         self.binary_name = binary_name
 
+    def met(self, session):
+        p = session.Popen(
+            ["which", self.binary_name], stdout=subprocess.DEVNULL,
+            stderr=subprocess.DEVNULL)
+        p.communicate()
+        return p.returncode == 0
+
 
 class PerlModuleRequirement(Requirement):
 
@@ -250,9 +278,15 @@ class HaskellPackageRequirement(Requirement):
 
     package: str
 
-    def __init__(self, package: str):
+    def __init__(self, package: str, specs=None):
         super(HaskellPackageRequirement, self).__init__("haskell-package")
         self.package = package
+        self.specs = specs
+
+    @classmethod
+    def from_string(cls, text):
+        parts = text.split()
+        return cls(parts[0], specs=parts[1:])
 
 
 class MavenArtifactRequirement(Requirement):
@@ -312,3 +346,22 @@ class PythonModuleRequirement(Requirement):
         super(PythonModuleRequirement, self).__init__("python-module")
         self.python_version = python_version
         self.minimum_version = minimum_version
+
+    def met(self, session):
+        if self.python_version == "cpython3":
+            cmd = "python3"
+        elif self.python_version == "cpython2":
+            cmd = "python2"
+        elif self.python_version == "pypy":
+            cmd = "pypy"
+        elif self.python_version == "pypy3":
+            cmd = "pypy3"
+        elif self.python_version is None:
+            cmd = "python3"
+        else:
+            raise NotImplementedError
+        p = session.Popen(
+            [cmd, "-c", "import %s" % self.module],
+            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+        p.communicate()
+        return p.returncode == 0
diff --git a/ognibuild/resolver/__init__.py b/ognibuild/resolver/__init__.py
index 8eca42c..0bd5c24 100644
--- a/ognibuild/resolver/__init__.py
+++ b/ognibuild/resolver/__init__.py
@@ -56,7 +56,6 @@ class CPANResolver(Resolver):
             # TODO(jelmer): Specify -T to skip tests?
             self.session.check_call(
                 ["cpan", "-i", requirement.module],
-                user="root",
                 env={"PERL_MM_USE_DEFAULT": "1"},
             )
         if missing:
@@ -85,7 +84,7 @@ class HackageResolver(Resolver):
                 missing.append(requirement)
                 continue
             self.session.check_call(
-                ["cabal", "install", requirement.package], user="root"
+                ["cabal", "install", requirement.package]
             )
         if missing:
             raise UnsatisfiedRequirements(missing)