This fixes rules_haskell to use the correct Windows path separator on Windows.

GHC expects the LD_LIBRARY_PATH variable to be a list of semi-colon separated
paths, as opposed to a list of colon separated paths:
https://github.com/ghc/ghc/blob/51fd357119b357c52e990ccce9059c423cc49406/compiler/ghci/Linker.hs#L1646-L1650

Drop this patch when this issue is closed:
https://github.com/tweag/rules_haskell/issues/810


diff --git a/haskell/private/path_utils.bzl b/haskell/private/path_utils.bzl
index db8ca35..e0af527 100644
--- a/haskell/private/path_utils.bzl
+++ b/haskell/private/path_utils.bzl
@@ -103,18 +103,21 @@ def declare_compiled(hs, src, ext, directory = None, rel_path = None):
 
     return hs.actions.declare_file(fp_with_dir)
 
-def make_path(libs, prefix = None):
+def make_path(libs, prefix = None, sep = None):
     """Return a string value for using as LD_LIBRARY_PATH or similar.
 
     Args:
       libs: List of library files that should be available
       prefix: String, an optional prefix to add to every path.
+      sep: String, the path separator, defaults to ":".
 
     Returns:
       String: paths to the given library directories separated by ":".
     """
     r = set.empty()
 
+    sep = sep if sep else ":"
+
     for lib in libs:
         lib_dir = paths.dirname(lib.path)
         if prefix:
@@ -122,7 +125,7 @@ def make_path(libs, prefix = None):
 
         set.mutable_insert(r, lib_dir)
 
-    return ":".join(set.to_list(r))
+    return sep.join(set.to_list(r))
 
 def darwin_convert_to_dylibs(hs, libs):
     """Convert .so dynamic libraries to .dylib.
diff --git a/haskell/private/providers.bzl b/haskell/private/providers.bzl
index c26ad95..bc35217 100644
--- a/haskell/private/providers.bzl
+++ b/haskell/private/providers.bzl
@@ -148,15 +148,20 @@ def get_libs_for_ghc_linker(hs, transitive_cc_dependencies, path_prefix = None):
         library_deps = _library_deps
         ld_library_deps = _ld_library_deps
 
+    sep = ";" if hs.toolchain.is_windows else None
+
     library_path = make_path(
         library_deps,
         prefix = path_prefix,
+        sep = sep,
     )
     ld_library_path = make_path(
         ld_library_deps,
         prefix = path_prefix,
+        sep = sep,
     )
 
+
     # GHC's builtin linker/loader looks for libraries in the paths defined by
     # LIBRARY_PATH and LD_LIBRARY_PATH.
     # See https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html?highlight=library_path#extra-libraries