From 196015799d3e0fcdc29d85408bbeac05fa4b1fd8 Mon Sep 17 00:00:00 2001
From: Federico Perini <federico.perini@gmail.com>
Date: Sat, 30 Aug 2025 09:19:22 +0200
Subject: [PATCH 1/6] add `module-dir` to the `install` table

---
 src/fpm/manifest/install.f90 | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/fpm/manifest/install.f90 b/src/fpm/manifest/install.f90
index 97634837fe..cc10c07e2b 100644
--- a/src/fpm/manifest/install.f90
+++ b/src/fpm/manifest/install.f90
@@ -4,11 +4,12 @@
 !>
 !>```toml
 !>library = bool
+!>module-dir = "path"
 !>```
 module fpm_manifest_install
   use fpm_error, only : error_t, fatal_error, syntax_error
   use tomlf, only : toml_table, toml_key, toml_stat
-  use fpm_toml, only : get_value, set_value, serializable_t
+  use fpm_toml, only : get_value, set_value, serializable_t, set_string
   implicit none
   private
 
@@ -23,6 +24,9 @@ module fpm_manifest_install
     !> Install tests with this project
     logical :: test = .false.
 
+    !> Directory where compiled module files should be installed
+    character(len=:), allocatable :: module_dir
+
   contains
 
     !> Print information on this instance
@@ -56,6 +60,7 @@ subroutine new_install_config(self, table, error)
 
     call get_value(table, "library", self%library, .false.)
     call get_value(table, "test", self%test, .false.)
+    call get_value(table, "module-dir", self%module_dir)
 
   end subroutine new_install_config
 
@@ -80,7 +85,7 @@ subroutine check(table, error)
       case default
         call syntax_error(error, "Key "//list(ikey)%key//" is not allowed in install table")
         exit
-      case("library","test")
+      case("library","test","module-dir")
         continue
       end select
     end do
@@ -114,6 +119,9 @@ subroutine info(self, unit, verbosity)
     write(unit, fmt) "Install configuration"
     write(unit, fmt) " - library install", trim(merge("enabled ", "disabled", self%library))
     write(unit, fmt) " - test    install", trim(merge("enabled ", "disabled", self%test))
+    if (allocated(self%module_dir)) then
+      write(unit, fmt) " - module directory", self%module_dir
+    end if
 
   end subroutine info
 
@@ -127,6 +135,10 @@ logical function install_conf_same(this,that)
        type is (install_config_t)
           if (this%library.neqv.other%library) return
           if (this%test.neqv.other%test) return
+          if (allocated(this%module_dir).neqv.allocated(other%module_dir)) return
+          if (allocated(this%module_dir)) then
+            if (.not.(this%module_dir==other%module_dir)) return
+          end if
        class default
           ! Not the same type
           return
@@ -155,6 +167,9 @@ subroutine dump_to_toml(self, table, error)
     call set_value(table, "test", self%test, error, class_name)
     if (allocated(error)) return
 
+    call set_string(table, "module-dir", self%module_dir, error, class_name)
+    if (allocated(error)) return
+
   end subroutine dump_to_toml
 
   !> Read install config from toml table (no checks made at this stage)
@@ -175,6 +190,8 @@ subroutine load_from_toml(self, table, error)
     if (allocated(error)) return
     call get_value(table, "test", self%test, error, class_name)
     if (allocated(error)) return
+    call get_value(table, "module-dir", self%module_dir)
+    if (allocated(error)) return
 
   end subroutine load_from_toml
 

From 6455ce44a6962876cbeca9129fad8eec6e9c6583 Mon Sep 17 00:00:00 2001
From: Federico Perini <federico.perini@gmail.com>
Date: Sat, 30 Aug 2025 09:25:20 +0200
Subject: [PATCH 2/6] manifest: add test

---
 test/fpm_test/test_manifest.f90 | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/test/fpm_test/test_manifest.f90 b/test/fpm_test/test_manifest.f90
index 42ab78cafa..5fd5b2a64b 100644
--- a/test/fpm_test/test_manifest.f90
+++ b/test/fpm_test/test_manifest.f90
@@ -68,6 +68,7 @@ subroutine collect_manifest(tests)
             & new_unittest("example-empty", test_example_empty, should_fail=.true.), &
             & new_unittest("install-library", test_install_library), &
             & new_unittest("install-empty", test_install_empty), &
+            & new_unittest("install-module-dir", test_install_module_dir), &
             & new_unittest("install-wrongkey", test_install_wrongkey, should_fail=.true.), &
             & new_unittest("preprocess-empty", test_preprocess_empty), &
             & new_unittest("preprocess-wrongkey", test_preprocess_wrongkey, should_fail=.true.), &
@@ -1359,6 +1360,34 @@ subroutine test_install_wrongkey(error)
 
     end subroutine test_install_wrongkey
 
+
+    subroutine test_install_module_dir(error)
+        use fpm_manifest_install
+
+        !> Error handling
+        type(error_t), allocatable, intent(out) :: error
+
+        type(toml_table) :: table
+        type(install_config_t) :: install
+
+        table = toml_table()
+        call set_value(table, "module-dir", "custom_modules")
+
+        call new_install_config(install, table, error)
+        if (allocated(error)) return
+
+        if (.not.allocated(install%module_dir)) then
+            call test_failed(error, "Module directory should be allocated")
+            return
+        end if
+
+        if (install%module_dir /= "custom_modules") then
+            call test_failed(error, "Module directory should match input")
+            return
+        end if
+
+    end subroutine test_install_module_dir
+
     subroutine test_preprocess_empty(error)
         use fpm_manifest_preprocess
 

From 388108d110757d3a5c2cbe138d0afef97a93cf48 Mon Sep 17 00:00:00 2001
From: Federico Perini <federico.perini@gmail.com>
Date: Sat, 30 Aug 2025 14:59:10 +0200
Subject: [PATCH 3/6] installer: use `moduledir` for modules

---
 src/fpm/cmd/install.f90          |  4 ++--
 src/fpm/installer.f90            | 29 ++++++++++++++++++++++++++++-
 test/fpm_test/test_installer.f90 | 17 +++++++++++++++++
 3 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/src/fpm/cmd/install.f90 b/src/fpm/cmd/install.f90
index 867959526c..0a33f044fb 100644
--- a/src/fpm/cmd/install.f90
+++ b/src/fpm/cmd/install.f90
@@ -63,7 +63,7 @@ subroutine cmd_install(settings)
 
     call new_installer(installer, prefix=settings%prefix, &
       bindir=settings%bindir, libdir=settings%libdir, testdir=settings%testdir, &
-      includedir=settings%includedir, &
+      includedir=settings%includedir, moduledir=package%install%module_dir, &
       verbosity=merge(2, 1, settings%verbose))
 
     if (allocated(package%library) .and. package%install%library) then
@@ -141,7 +141,7 @@ subroutine install_module_files(installer, targets, error)
     call filter_modules(targets, modules)
 
     do ii = 1, size(modules)
-      call installer%install_header(modules(ii)%s//".mod", error)
+      call installer%install_module(modules(ii)%s//".mod", error)
       if (allocated(error)) exit
     end do
     if (allocated(error)) return
diff --git a/src/fpm/installer.f90 b/src/fpm/installer.f90
index 42cba10b91..704c10a243 100644
--- a/src/fpm/installer.f90
+++ b/src/fpm/installer.f90
@@ -27,6 +27,8 @@ module fpm_installer
     character(len=:), allocatable :: testdir
     !> Include directory relative to the installation prefix
     character(len=:), allocatable :: includedir
+    !> Module directory relative to the installation prefix
+    character(len=:), allocatable :: moduledir
     !> Output unit for informative printout
     integer :: unit = output_unit
     !> Verbosity of the installer
@@ -46,6 +48,8 @@ module fpm_installer
     procedure :: install_library
     !> Install a header/module in its correct subdirectory
     procedure :: install_header
+    !> Install a module in its correct subdirectory
+    procedure :: install_module
     !> Install a test program in its correct subdirectory
     procedure :: install_test
     !> Install a generic file into a subdirectory in the installation prefix
@@ -69,6 +73,9 @@ module fpm_installer
   !> Default name of the include subdirectory
   character(len=*), parameter :: default_includedir = "include"
 
+  !> Default name of the module subdirectory
+  character(len=*), parameter :: default_moduledir = "include"
+
   !> Copy command on Unix platforms
   character(len=*), parameter :: default_copy_unix = "cp"
 
@@ -90,7 +97,7 @@ module fpm_installer
 contains
 
   !> Create a new instance of an installer
-  subroutine new_installer(self, prefix, bindir, libdir, includedir, testdir, verbosity, &
+  subroutine new_installer(self, prefix, bindir, libdir, includedir, moduledir, testdir, verbosity, &
           copy, move)
     !> Instance of the installer
     type(installer_t), intent(out) :: self
@@ -102,6 +109,8 @@ subroutine new_installer(self, prefix, bindir, libdir, includedir, testdir, verb
     character(len=*), intent(in), optional :: libdir
     !> Include directory relative to the installation prefix
     character(len=*), intent(in), optional :: includedir
+    !> Module directory relative to the installation prefix
+    character(len=*), intent(in), optional :: moduledir
     !> Test directory relative to the installation prefix
     character(len=*), intent(in), optional :: testdir    
     !> Verbosity of the installer
@@ -139,6 +148,12 @@ subroutine new_installer(self, prefix, bindir, libdir, includedir, testdir, verb
     else
       self%includedir = default_includedir
     end if
+
+    if (present(moduledir)) then
+      self%moduledir = moduledir
+    else
+      self%moduledir = default_moduledir
+    end if
     
     if (present(testdir)) then 
       self%testdir = testdir
@@ -288,6 +303,18 @@ subroutine install_header(self, header, error)
     call self%install(header, self%includedir, error)
   end subroutine install_header
 
+  !> Install a module in its correct subdirectory
+  subroutine install_module(self, module, error)
+    !> Instance of the installer
+    class(installer_t), intent(inout) :: self
+    !> Path to the module
+    character(len=*), intent(in) :: module
+    !> Error handling
+    type(error_t), allocatable, intent(out) :: error
+
+    call self%install(module, self%moduledir, error)
+  end subroutine install_module
+
   !> Install a generic file into a subdirectory in the installation prefix
   subroutine install(self, source, destination, error)
     !> Instance of the installer
diff --git a/test/fpm_test/test_installer.f90 b/test/fpm_test/test_installer.f90
index edc5838d88..1d47ed2e41 100644
--- a/test/fpm_test/test_installer.f90
+++ b/test/fpm_test/test_installer.f90
@@ -36,6 +36,7 @@ subroutine collect_installer(testsuite)
             & new_unittest("install-pkgconfig", test_install_pkgconfig), &
             & new_unittest("install-sitepackages", test_install_sitepackages), &
             & new_unittest("install-mod", test_install_mod), &
+            & new_unittest("install-module-custom", test_install_module_custom), &
             & new_unittest("install-exe-unix", test_install_exe_unix), &
             & new_unittest("install-exe-win", test_install_exe_win), &
             & new_unittest("install-test-unix", test_install_tests_unix), &
@@ -184,6 +185,22 @@ subroutine test_install_mod(error)
 
     end subroutine test_install_mod
 
+    subroutine test_install_module_custom(error)
+        !> Error handling
+        type(error_t), allocatable, intent(out) :: error
+
+        type(mock_installer_t) :: mock
+        type(installer_t) :: installer
+
+        call new_installer(installer, prefix="PREFIX", moduledir="custom/modules", verbosity=0, copy="mock")
+        mock%installer_t = installer
+        mock%expected_dir = join_path("PREFIX", "custom/modules")
+        mock%expected_run = 'mock "test_module.mod" "'//join_path("PREFIX", "custom/modules")//'"'
+
+        call mock%install_module("test_module.mod", error)
+
+    end subroutine test_install_module_custom
+
     subroutine test_install_shared_library_unix(error)
         !> Error handling
         type(error_t), allocatable, intent(out) :: error

From 01872bd1abc2f968d26754c3a79cb8aca77064aa Mon Sep 17 00:00:00 2001
From: Federico Perini <federico.perini@gmail.com>
Date: Sat, 30 Aug 2025 15:09:51 +0200
Subject: [PATCH 4/6] add example package

---
 example_packages/custom_module_dir/README.md  | 32 +++++++++++++++++++
 example_packages/custom_module_dir/fpm.toml   |  3 ++
 .../custom_module_dir/src/greeting.f90        | 13 ++++++++
 .../custom_module_dir/src/math_utils.f90      | 20 ++++++++++++
 4 files changed, 68 insertions(+)
 create mode 100644 example_packages/custom_module_dir/README.md
 create mode 100644 example_packages/custom_module_dir/fpm.toml
 create mode 100644 example_packages/custom_module_dir/src/greeting.f90
 create mode 100644 example_packages/custom_module_dir/src/math_utils.f90

diff --git a/example_packages/custom_module_dir/README.md b/example_packages/custom_module_dir/README.md
new file mode 100644
index 0000000000..92790bb6a2
--- /dev/null
+++ b/example_packages/custom_module_dir/README.md
@@ -0,0 +1,32 @@
+# Custom Module Directory Example
+
+This example demonstrates the use of a custom module directory in the `[install]` section of `fpm.toml`.
+
+## Features
+
+- Two simple Fortran modules: `greeting` and `math_utils`
+- Custom module installation directory specified as `custom/modules`
+- Shows how modules can be installed to a different location than headers
+
+## Configuration
+
+In `fpm.toml`:
+
+```toml
+[install]
+library = true
+module-dir = "custom/modules"
+```
+
+This configuration will install compiled `.mod` files to the `custom/modules` directory instead of the default `include` directory.
+
+## Testing
+
+To test this example:
+
+```bash
+cd example_packages/custom_module_dir
+fpm build
+fpm install --prefix /tmp/test_install
+# Check that .mod files are in /tmp/test_install/custom/modules/
+```
\ No newline at end of file
diff --git a/example_packages/custom_module_dir/fpm.toml b/example_packages/custom_module_dir/fpm.toml
new file mode 100644
index 0000000000..1f51b240f1
--- /dev/null
+++ b/example_packages/custom_module_dir/fpm.toml
@@ -0,0 +1,3 @@
+name = "custom-module-dir"
+install.library = true
+install.module-dir = "custom/modules"
diff --git a/example_packages/custom_module_dir/src/greeting.f90 b/example_packages/custom_module_dir/src/greeting.f90
new file mode 100644
index 0000000000..3cb2b20266
--- /dev/null
+++ b/example_packages/custom_module_dir/src/greeting.f90
@@ -0,0 +1,13 @@
+module greeting
+    implicit none
+    private
+    public :: say_hello
+
+contains
+
+    subroutine say_hello(name)
+        character(len=*), intent(in) :: name
+        print *, 'Hello, ' // name // '!'
+    end subroutine say_hello
+
+end module greeting
\ No newline at end of file
diff --git a/example_packages/custom_module_dir/src/math_utils.f90 b/example_packages/custom_module_dir/src/math_utils.f90
new file mode 100644
index 0000000000..c8915dd630
--- /dev/null
+++ b/example_packages/custom_module_dir/src/math_utils.f90
@@ -0,0 +1,20 @@
+module math_utils
+    implicit none
+    private
+    public :: add_numbers, multiply_numbers
+
+contains
+
+    function add_numbers(a, b) result(sum)
+        integer, intent(in) :: a, b
+        integer :: sum
+        sum = a + b
+    end function add_numbers
+
+    function multiply_numbers(a, b) result(product)
+        integer, intent(in) :: a, b
+        integer :: product
+        product = a * b
+    end function multiply_numbers
+
+end module math_utils
\ No newline at end of file

From f01f9cb1aaf9e1bc4884347049984302d08c2afe Mon Sep 17 00:00:00 2001
From: Federico Perini <federico.perini@gmail.com>
Date: Sat, 30 Aug 2025 15:09:55 +0200
Subject: [PATCH 5/6] add CI test

---
 ci/run_tests.sh | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/ci/run_tests.sh b/ci/run_tests.sh
index 0260743f16..fbf147fe4f 100755
--- a/ci/run_tests.sh
+++ b/ci/run_tests.sh
@@ -344,5 +344,19 @@ pushd static_app_only
 test $EXIT_CODE -eq 0
 popd
 
+# Test custom module directory
+pushd custom_module_dir
+"$fpm" build
+rm -rf ./test_custom_install
+"$fpm" install --prefix ./test_custom_install
+# Verify modules are installed in custom directory
+test -f ./test_custom_install/custom/modules/greeting.mod
+test -f ./test_custom_install/custom/modules/math_utils.mod
+# Verify library is still installed normally
+test -f ./test_custom_install/lib/libcustom-module-dir.a
+# Clean up
+rm -rf ./test_custom_install
+popd
+
 # Cleanup
 rm -rf ./*/build

From 42ca4d0c84f406851a49a15228d246ba648954da Mon Sep 17 00:00:00 2001
From: Federico Perini <federico.perini@gmail.com>
Date: Sat, 30 Aug 2025 15:33:19 +0200
Subject: [PATCH 6/6] do not use slash in test (windows compliant)

---
 ci/run_tests.sh                              | 4 ++--
 example_packages/custom_module_dir/README.md | 8 ++++----
 example_packages/custom_module_dir/fpm.toml  | 2 +-
 test/fpm_test/test_installer.f90             | 6 +++---
 4 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/ci/run_tests.sh b/ci/run_tests.sh
index fbf147fe4f..72f666d4a9 100755
--- a/ci/run_tests.sh
+++ b/ci/run_tests.sh
@@ -350,8 +350,8 @@ pushd custom_module_dir
 rm -rf ./test_custom_install
 "$fpm" install --prefix ./test_custom_install
 # Verify modules are installed in custom directory
-test -f ./test_custom_install/custom/modules/greeting.mod
-test -f ./test_custom_install/custom/modules/math_utils.mod
+test -f ./test_custom_install/custom_modules/greeting.mod
+test -f ./test_custom_install/custom_modules/math_utils.mod
 # Verify library is still installed normally
 test -f ./test_custom_install/lib/libcustom-module-dir.a
 # Clean up
diff --git a/example_packages/custom_module_dir/README.md b/example_packages/custom_module_dir/README.md
index 92790bb6a2..76ffa5d25d 100644
--- a/example_packages/custom_module_dir/README.md
+++ b/example_packages/custom_module_dir/README.md
@@ -5,7 +5,7 @@ This example demonstrates the use of a custom module directory in the `[install]
 ## Features
 
 - Two simple Fortran modules: `greeting` and `math_utils`
-- Custom module installation directory specified as `custom/modules`
+- Custom module installation directory specified as `custom_modules`
 - Shows how modules can be installed to a different location than headers
 
 ## Configuration
@@ -15,10 +15,10 @@ In `fpm.toml`:
 ```toml
 [install]
 library = true
-module-dir = "custom/modules"
+module-dir = "custom_modules"
 ```
 
-This configuration will install compiled `.mod` files to the `custom/modules` directory instead of the default `include` directory.
+This configuration will install compiled `.mod` files to the `custom_modules` directory instead of the default `include` directory.
 
 ## Testing
 
@@ -28,5 +28,5 @@ To test this example:
 cd example_packages/custom_module_dir
 fpm build
 fpm install --prefix /tmp/test_install
-# Check that .mod files are in /tmp/test_install/custom/modules/
+# Check that .mod files are in /tmp/test_install/custom_modules/
 ```
\ No newline at end of file
diff --git a/example_packages/custom_module_dir/fpm.toml b/example_packages/custom_module_dir/fpm.toml
index 1f51b240f1..9487cc1268 100644
--- a/example_packages/custom_module_dir/fpm.toml
+++ b/example_packages/custom_module_dir/fpm.toml
@@ -1,3 +1,3 @@
 name = "custom-module-dir"
 install.library = true
-install.module-dir = "custom/modules"
+install.module-dir = "custom_modules"
diff --git a/test/fpm_test/test_installer.f90 b/test/fpm_test/test_installer.f90
index 1d47ed2e41..1a7e3641f2 100644
--- a/test/fpm_test/test_installer.f90
+++ b/test/fpm_test/test_installer.f90
@@ -192,10 +192,10 @@ subroutine test_install_module_custom(error)
         type(mock_installer_t) :: mock
         type(installer_t) :: installer
 
-        call new_installer(installer, prefix="PREFIX", moduledir="custom/modules", verbosity=0, copy="mock")
+        call new_installer(installer, prefix="PREFIX", moduledir="custom_modules", verbosity=0, copy="mock")
         mock%installer_t = installer
-        mock%expected_dir = join_path("PREFIX", "custom/modules")
-        mock%expected_run = 'mock "test_module.mod" "'//join_path("PREFIX", "custom/modules")//'"'
+        mock%expected_dir = join_path("PREFIX", "custom_modules")
+        mock%expected_run = 'mock "test_module.mod" "'//join_path("PREFIX", "custom_modules")//'"'
 
         call mock%install_module("test_module.mod", error)