Windows下GHCi无法加载lmdb.dll：Haskell LMDB绑定库调用异常求助

Hey there, let's tackle this problem step by step—your error is a classic case of GHCi not finding the native LMDB DLL, plus a potential mismatch in how the Haskell bindings were linked to the library. Here's what you can do to fix it, tailored to your Windows 7 x64, Haskell Platform 8.0.2, and vcpkg setup:

1. Add LMDB's DLL path to your system PATH #

vcpkg installs the 64-bit lmdb.dll in C:\vcpkg\installed\x64-windows\bin (adjust the path if you installed vcpkg elsewhere). GHCi relies on the system PATH to locate native DLLs, so you need to add this directory:

• Right-click "Computer" → Properties → Advanced System Settings → Environment Variables
• Under System Variables, find PATH, click Edit, and add the full path to the bin folder
• Restart your terminal/IDE—this is crucial for the PATH change to take effect

2. Recompile the Haskell LMDB bindings with explicit library paths #

Sometimes Cabal doesn't automatically pick up vcpkg's installed libraries. Force it to use the vcpkg LMDB by specifying the include and lib directories during installation:

cabal install lmdb --extra-include-dirs=C:\vcpkg\installed\x64-windows\include --extra-lib-dirs=C:\vcpkg\installed\x64-windows\lib

This ensures the bindings are compiled against the correct 64-bit LMDB library from vcpkg, avoiding conflicts with any 32-bit versions that might be lurking on your system.

3. Manually load the DLL in GHCi (if PATH changes don't stick) #

If restarting your terminal still doesn't work, you can directly load the DLL in GHCi before using your module:

Prelude> import System.Win32.DLL
Prelude System.Win32.DLL> loadLibrary "C:\\vcpkg\\installed\\x64-windows\\bin\\lmdb.dll"

Once it loads successfully, import your Persistence module and try calling lmdb_version again.

4. Verify 32/64-bit compatibility #

Double-check that your Haskell Platform is 64-bit—if it's 32-bit, it won't work with the x64-windows LMDB from vcpkg. Run this command in your terminal:

ghc --version

Look for x86_64 in the output (that means 64-bit). If you see i386, you'll need to:

• Uninstall the x64 LMDB via vcpkg: vcpkg remove lmdb:x64-windows
• Install the 32-bit version: vcpkg install lmdb:x86-windows
• Recompile the Haskell bindings with the 32-bit paths

5. Clean old Cabal caches #

Stale compilation artifacts can cause weird symbol errors. Clear them out and reinstall the bindings fresh:

cabal clean
rmdir /s /q C:\Users\thanhdo\AppData\Roaming\cabal\x86_64-windows-ghc-8.0.2\lmdb-*
cabal install lmdb --extra-include-dirs=C:\vcpkg\installed\x64-windows\include --extra-lib-dirs=C:\vcpkg\installed\x64-windows\lib

Work through these steps one by one, and you should be able to get the LMDB bindings working in GHCi without those DLL and symbol errors.

内容的提问来源于stack exchange，提问作者T. Do