diff --git a/pkgs/development/compilers/fsharp41/default.nix b/pkgs/development/compilers/fsharp41/default.nix
index e0094c73098f..468fd49ba44e 100644
--- a/pkgs/development/compilers/fsharp41/default.nix
+++ b/pkgs/development/compilers/fsharp41/default.nix
@@ -1,14 +1,14 @@
 # Temporaririly avoid dependency on dotnetbuildhelpers to avoid rebuilding many times while working on it
 
-{ stdenv, fetchurl, pkgconfig, autoconf, automake, which, mono, dotnetbuildhelpers, dotnetPackages }:
+{ stdenv, fetchurl, pkgconfig, autoconf, automake, which, mono, msbuild, dotnetbuildhelpers, dotnetPackages }:
 
 stdenv.mkDerivation rec {
   name = "fsharp-${version}";
-  version = "4.1.7";
+  version = "4.1.34";
 
   src = fetchurl {
     url = "https://github.com/fsharp/fsharp/archive/${version}.tar.gz";
-    sha256 = "0rfkrk4mzi4w54mfqilvng9ar5swhmnwhsyjc54rx3fd0np3jiyl";
+    sha256 = "0cv6p5pin962vhbpsji40nkckkag5c96kq5qihvg60pc1z821p0i";
   };
 
   nativeBuildInputs = [ pkgconfig ];
@@ -17,12 +17,16 @@ stdenv.mkDerivation rec {
     automake
     which
     mono
+    msbuild
     dotnetbuildhelpers
     dotnetPackages.FsCheck262
     dotnetPackages.FSharpCompilerTools
-    dotnetPackages.FSharpCore
+    dotnetPackages.FSharpCore302
+    dotnetPackages.FSharpCore3125
+    dotnetPackages.FSharpCore4001
+    dotnetPackages.FSharpCore4117
     dotnetPackages.FSharpData225
-    dotnetPackages.FsLexYacc704
+    dotnetPackages.FsLexYacc706
     dotnetPackages.MicrosoftDiaSymReader
     dotnetPackages.MicrosoftDiaSymReaderPortablePdb
     dotnetPackages.NUnit350
@@ -31,6 +35,14 @@ stdenv.mkDerivation rec {
     dotnetPackages.SystemValueTuple
   ];
 
+  # https://github.com/mono/mono/tree/fe0f311a848068ab2d17a9b9dd15326e5712d520/packaging/MacSDK/patches
+  # https://github.com/mono/mono/issues/7805
+  patches = [
+    ./fsharp-IsPathRooted-type-inference.patch
+    ./fsharp-string-switchName.patch
+    ./fsharp-path-overloads.patch
+  ];
+
   configurePhase = ''
     substituteInPlace ./autogen.sh --replace "/usr/bin/env sh" "${stdenv.shell}"
     ./autogen.sh --prefix $out
@@ -47,24 +59,30 @@ stdenv.mkDerivation rec {
     mkdir packages
 
     ln -s ${dotnetPackages.FsCheck262}/lib/dotnet/FsCheck packages/FsCheck.2.6.2
-    ln -s ${dotnetPackages.FSharpCompilerTools}/lib/dotnet/FSharp.Compiler.Tools packages/FSharp.Compiler.Tools.4.1.4
-    ln -s ${dotnetPackages.FSharpCore}/lib/dotnet/FSharp.Core/ packages/FSharp.Core.4.0.0.1
+    ln -s ${dotnetPackages.FSharpCompilerTools}/lib/dotnet/FSharp.Compiler.Tools packages/FSharp.Compiler.Tools.4.1.27
+    ln -s ${dotnetPackages.FSharpCore302}/lib/dotnet/FSharp.Core/ packages/FSharp.Core.3.0.2
+    ln -s ${dotnetPackages.FSharpCore3125}/lib/dotnet/FSharp.Core/ packages/FSharp.Core.3.1.2.5
+    ln -s ${dotnetPackages.FSharpCore4001}/lib/dotnet/FSharp.Core/ packages/FSharp.Core.4.0.0.1
+    ln -s ${dotnetPackages.FSharpCore4117}/lib/dotnet/FSharp.Core/ packages/FSharp.Core.4.1.17
     ln -s ${dotnetPackages.FSharpData225}/lib/dotnet/FSharp.Data/ packages/FSharp.Data.2.2.5
-    ln -s ${dotnetPackages.FsLexYacc704}/lib/dotnet/FsLexYacc/ packages/FsLexYacc.7.0.4
+    ln -s ${dotnetPackages.FsLexYacc706}/lib/dotnet/FsLexYacc/ packages/FsLexYacc.7.0.6
     ln -s ${dotnetPackages.MicrosoftDiaSymReader}/lib/dotnet/Microsoft.DiaSymReader/ packages/Microsoft.DiaSymReader.1.1.0
     ln -s ${dotnetPackages.MicrosoftDiaSymReaderPortablePdb}/lib/dotnet/Microsoft.DiaSymReader.PortablePdb/ packages/Microsoft.DiaSymReader.PortablePdb.1.2.0
     ln -s ${dotnetPackages.NUnit350}/lib/dotnet/NUnit/ packages/NUnit.3.5.0
     ln -s ${dotnetPackages.SystemCollectionsImmutable131}/lib/dotnet/System.Collections.Immutable/ packages/System.Collections.Immutable.1.3.1
     ln -s ${dotnetPackages.SystemReflectionMetadata}/lib/dotnet/System.Reflection.Metadata/ packages/System.Reflection.Metadata.1.4.2
-    ln -s ${dotnetPackages.SystemValueTuple}/lib/dotnet/System.ValueTuple/ packages/System.ValueTuple.4.3.0
+    ln -s ${dotnetPackages.SystemValueTuple}/lib/dotnet/System.ValueTuple/ packages/System.ValueTuple.4.3.1
   '';
 
-  # Make sure the executables use the right mono binary,
-  # and set up some symlinks for backwards compatibility.
+  # Signing /home/jdanek/nix/nixpkgs/build/fss/fsharp-4.1.34/again/fsharp-4.1.34/Release/fsharp30/net40/bin/FSharp.Core.dll with Mono key
+  # ERROR: Unknown error during processing: System.UnauthorizedAccessException: Access to the path
+  #   "Release/fsharp30/net40/bin/FSharp.Core.dll" is denied.
+  preInstall = ''
+    find Release/ -name FSharp.Core.dll -exec chmod u+w {} \;
+  '';
+
+  # Set up some symlinks for backwards compatibility.
   postInstall = ''
-    substituteInPlace $out/bin/fsharpc --replace " mono " " ${mono}/bin/mono "
-    substituteInPlace $out/bin/fsharpi --replace " mono " " ${mono}/bin/mono "
-    substituteInPlace $out/bin/fsharpiAnyCpu --replace " mono " " ${mono}/bin/mono "
     ln -s $out/bin/fsharpc $out/bin/fsc
     ln -s $out/bin/fsharpi $out/bin/fsi
     for dll in "$out/lib/mono/fsharp"/FSharp*.dll
@@ -73,6 +91,26 @@ stdenv.mkDerivation rec {
     done
   '';
 
+  doInstallCheck = true;
+  installCheckPhase = ''
+    echo 'printf "int = %i" (6 * 7);;' > script.fsx
+    $out/bin/fsi --exec script.fsx | grep "int = 42"
+    $out/bin/fsharpi --exec script.fsx | grep "int = 42"
+    $out/bin/fsharpiAnyCpu --exec script.fsx | grep "int = 42"
+
+    cat > answer.fs <<EOF
+open System
+
+[<EntryPoint>]
+let main argv =
+    printfn "int = %i" (6 * 7)
+    0
+EOF
+
+    $out/bin/fsc answer.fs
+    ${mono}/bin/mono answer.exe | grep "int = 42"
+  '';
+
   # To fix this error when running:
   # The file "/nix/store/path/whatever.exe" is an not a valid CIL image
   dontStrip = true;
diff --git a/pkgs/development/compilers/fsharp41/fsharp-IsPathRooted-type-inference.patch b/pkgs/development/compilers/fsharp41/fsharp-IsPathRooted-type-inference.patch
new file mode 100644
index 000000000000..06dd3e82adc9
--- /dev/null
+++ b/pkgs/development/compilers/fsharp41/fsharp-IsPathRooted-type-inference.patch
@@ -0,0 +1,21 @@
+commit c37fce5b3019c7a150203fc3a484885591b194de
+Author: Alexis Christoforides <alexis@thenull.net>
+Date:   Sun Dec 2 00:10:24 2018 -0500
+
+    Help Path.IsPathRooted method overload selection.
+
+    .NET Core, and Mono after merging https://github.com/mono/mono/pull/11342, introduce ambiguity with a new overload.
+
+diff --git a/src/scripts/scriptlib.fsx b/src/scripts/scriptlib.fsx
+index cc797e305..699c7bb93 100644
+--- a/src/scripts/scriptlib.fsx
++++ b/src/scripts/scriptlib.fsx
+@@ -92,7 +92,7 @@ module Scripting =
+
+     module Process =
+
+-        let processExePath baseDir exe =
++        let processExePath baseDir (exe:string) =
+             if Path.IsPathRooted(exe) then exe
+             else
+                 match Path.GetDirectoryName(exe) with
diff --git a/pkgs/development/compilers/fsharp41/fsharp-path-overloads.patch b/pkgs/development/compilers/fsharp41/fsharp-path-overloads.patch
new file mode 100644
index 000000000000..f791317d0802
--- /dev/null
+++ b/pkgs/development/compilers/fsharp41/fsharp-path-overloads.patch
@@ -0,0 +1,22 @@
+diff --git a/src/scripts/scriptlib.fsx b/src/scripts/scriptlib.fsx
+index cc797e305..ae8a6d3cc 100644
+--- a/src/scripts/scriptlib.fsx
++++ b/src/scripts/scriptlib.fsx
+@@ -59,12 +59,12 @@ module Scripting =
+
+     let (++) a b = Path.Combine(a,b)
+
+-    let getBasename a = Path.GetFileNameWithoutExtension a
+-    let getFullPath a = Path.GetFullPath a
+-    let getFilename a = Path.GetFileName a
+-    let getDirectoryName a = Path.GetDirectoryName a
++    let getBasename (path: string) = Path.GetFileNameWithoutExtension path
++    let getFullPath (path: string) = Path.GetFullPath path
++    let getFilename (path: string) = Path.GetFileName path
++    let getDirectoryName (path: string) = Path.GetDirectoryName path
+
+-    let copyFile source dir =
++    let copyFile (source: string) dir =
+         let dest = 
+             if not (Directory.Exists dir) then Directory.CreateDirectory dir |>ignore
+             let result = Path.Combine(dir, Path.GetFileName source)
diff --git a/pkgs/development/compilers/fsharp41/fsharp-string-switchName.patch b/pkgs/development/compilers/fsharp41/fsharp-string-switchName.patch
new file mode 100644
index 000000000000..4b36eaabcafc
--- /dev/null
+++ b/pkgs/development/compilers/fsharp41/fsharp-string-switchName.patch
@@ -0,0 +1,13 @@
+diff --git a/src/scripts/scriptlib.fsx b/src/scripts/scriptlib.fsx
+index cc797e305..5a7be7d2b 100644
+--- a/src/scripts/scriptlib.fsx
++++ b/src/scripts/scriptlib.fsx
+@@ -36,7 +36,7 @@ module Scripting =
+ #if INTERACTIVE
+     let argv = Microsoft.FSharp.Compiler.Interactive.Settings.fsi.CommandLineArgs |> Seq.skip 1 |> Seq.toArray
+
+-    let getCmdLineArgOptional switchName = 
++    let getCmdLineArgOptional (switchName: string) =
+         argv |> Array.filter(fun t -> t.StartsWith(switchName)) |> Array.map(fun t -> t.Remove(0, switchName.Length).Trim()) |> Array.tryHead 
+
+     let getCmdLineArg switchName defaultValue = 
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 37e9f6bd7348..a8e21212c99d 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -7714,7 +7714,7 @@ in
 
   fsharp = callPackage ../development/compilers/fsharp { };
 
-  fsharp41 = callPackage ../development/compilers/fsharp41 { mono = mono4; };
+  fsharp41 = callPackage ../development/compilers/fsharp41 { mono = mono6; };
 
   fstar = callPackage ../development/compilers/fstar { };
 
diff --git a/pkgs/top-level/dotnet-packages.nix b/pkgs/top-level/dotnet-packages.nix
index 1da3968cd9fa..e89a7d9df2aa 100644
--- a/pkgs/top-level/dotnet-packages.nix
+++ b/pkgs/top-level/dotnet-packages.nix
@@ -45,10 +45,31 @@ let self = dotnetPackages // overrides; dotnetPackages = with self; {
     outputFiles = [ "lib/net40/*" ];
   };
 
-  FSharpCore = fetchNuGet {
+  FSharpCore302 = fetchNuGet {
+    baseName = "FSharp.Core";
+    version = "3.0.2";
+    sha256 = "1s4pqwbmhrsg5sw8i6dixdri3x0yjyilmkhsf4apfkp80si7d73q";
+    outputFiles = [ "*" ];
+  };
+
+  FSharpCore3125 = fetchNuGet {
+    baseName = "FSharp.Core";
+    version = "3.1.2.5";
+    sha256 = "0pfvjimrgrffb5rj612gsid044lfpk8g2cxyh9792dc1n8ck5hih";
+    outputFiles = [ "*" ];
+  };
+
+  FSharpCore4001 = fetchNuGet {
     baseName = "FSharp.Core";
     version = "4.0.0.1";
-    sha256 = "01nhjcxdz8l1r5vvdzhmgy5x7z5fqppab3ki34qg14axgf8jjygn";
+    sha256 = "0v53iq12ji2d1bkdyg9dn8sz5l93sprrh835amh39dghh8v8vm8k";
+    outputFiles = [ "*" ];
+  };
+
+  FSharpCore4117 = fetchNuGet {
+    baseName = "FSharp.Core";
+    version = "4.1.17";
+    sha256 = "1yk23ir66fgqm5r6qyf66zf64l0s223l3yd7p9yvbyimyg0hgzb1";
     outputFiles = [ "*" ];
   };
 
@@ -89,8 +110,8 @@ let self = dotnetPackages // overrides; dotnetPackages = with self; {
 
   FSharpCompilerTools = fetchNuGet {
     baseName = "FSharp.Compiler.Tools";
-    version = "4.1.4";
-    sha256 = "0vsp0khlnwh15ibg8s161rw6a6i8rlriclpq53paga447jllf0m8";
+    version = "4.1.27";
+    sha256 = "1m3hl8ja9gp5ajxmjf7bnq24bbkd6kx7yhxf4zb8si27h1n9l6dl";
     outputFiles = [ "*" ];
   };
 
@@ -101,10 +122,10 @@ let self = dotnetPackages // overrides; dotnetPackages = with self; {
     outputFiles = [ "build/*" ];
   };
 
-  FsLexYacc704 = fetchNuGet {
+  FsLexYacc706 = fetchNuGet {
     baseName = "FsLexYacc";
-    version = "7.0.4";
-    sha256 = "01zpdb0pybdf0by02rwd7pb1g0cmnn8jxm2pibzxjxw6f4l43ywi";
+    version = "7.0.6";
+    sha256 = "0xwiq8q5q6ga6zj24w83ch5csbv405xcg6jg2hmnjic0npz0drk2";
     outputFiles = [ "*" ];
   };
 
@@ -203,8 +224,8 @@ let self = dotnetPackages // overrides; dotnetPackages = with self; {
 
   SystemValueTuple = fetchNuGet {
     baseName = "System.ValueTuple";
-    version = "4.3.0";
-    sha256 = "00p5s753xh5417arw3k6npf1pc1k3m1s9mrlkw5vmc7pg8lm6n88";
+    version = "4.3.1";
+    sha256 = "0qzq878s66yfkf4n2b9af8lw2bx45s3cg6mi0w8w0bi358fa7q70";
     outputFiles = [ "*" ];
   };