Compiling with GHC fails with error: No instance for ToJSString (Maybe [Char])

[hansroland]

I tried to compile ghcjs-dom-hello under GHC / WebkitGtk2 with the following stack.yaml file:

resolver: lts-8.17
packages:
- location: '.'
#- location:
#    git: https://github.com/GHCJS/jsaddle
#    commit: a51df99cbcca19d11df079f1a99771043153d1a7 
#  subdirs:
#  - jsaddle
# extra-dep: true
  
extra-deps:
- jsaddle-0.9.0.0
- jsaddle-dom-0.9.0.0
- ghcjs-dom-0.9.0.0
- jsaddle-warp-0.9.0.0
- jsaddle-webkit2gtk-0.9.0.0
- ref-tf-0.4.0.1
- ghcjs-dom-jsaddle-0.9.0.0
- gi-webkit2-4.0.12
- webkit2gtk3-javascriptcore-0.14.2.1
- gi-javascriptcore-4.0.12
flags: {}

extra-package-dbs: []

And got the error:

/home/roland/Projekte/ghcjs-dom-hello/src/HelloMain.hs:22:5-52: error:
    • No instance for (Language.Javascript.JSaddle.Marshal.String.ToJSString
                         (Maybe [Char]))
        arising from a use of ‘setInnerHTML’

Note: To be able to build under Linux I had to remove the section executable ghcjs-dom-hello-wkwebview and the if os(osx) || os(ios) from the cabal file.

I don't really understand why we have this error. We have in module Language.Javascript.JSaddle.Marshal.String :

-- | Anything that can be used to make a JavaScript string
class ToJSVal a => ToJSString a where
    toJSString :: a -> JSString

and in module Language.Javascript.JSaddle.Value :

instance ToJSVal a => ToJSVal (Maybe a) where
    toJSVal Nothing = return jsNull
    toJSVal (Just a) = toJSVal a

Here is the full building log with the complete error message:

roland@goms:~/Projekte/ghcjs-dom-hello$ stack setup
stack will use a sandboxed GHC it installed
For more information on paths, see 'stack path' and 'stack exec env'
To use this GHC and packages outside of a project, consider using:
stack ghc, stack ghci, stack runghc, or stack exec
roland@goms:~/Projekte/ghcjs-dom-hello$ stack setup
stack will use a sandboxed GHC it installed
For more information on paths, see 'stack path' and 'stack exec env'
To use this GHC and packages outside of a project, consider using:
stack ghc, stack ghci, stack runghc, or stack exec
roland@goms:~/Projekte/ghcjs-dom-hello$ stack build
webkit2gtk3-javascriptcore-0.14.2.1: configure
webkit2gtk3-javascriptcore-0.14.2.1: build
gi-javascriptcore-4.0.12: configure
webkit2gtk3-javascriptcore-0.14.2.1: copy/register
gi-javascriptcore-4.0.12: build
ref-tf-0.4.0.1: configure
ref-tf-0.4.0.1: build
gi-javascriptcore-4.0.12: copy/register
ref-tf-0.4.0.1: copy/register
gi-webkit2-4.0.12: configure
gi-webkit2-4.0.12: build
jsaddle-0.9.0.0: configure
jsaddle-0.9.0.0: build
gi-webkit2-4.0.12: copy/register
jsaddle-0.9.0.0: copy/register
jsaddle-warp-0.9.0.0: configure
jsaddle-warp-0.9.0.0: build
jsaddle-webkit2gtk-0.9.0.0: configure
jsaddle-webkit2gtk-0.9.0.0: build
jsaddle-dom-0.9.0.0: configure
jsaddle-dom-0.9.0.0: build
jsaddle-warp-0.9.0.0: copy/register
jsaddle-webkit2gtk-0.9.0.0: copy/register
jsaddle-dom-0.9.0.0: copy/register
ghcjs-dom-jsaddle-0.9.0.0: configure
ghcjs-dom-jsaddle-0.9.0.0: build
ghcjs-dom-jsaddle-0.9.0.0: copy/register
ghcjs-dom-0.9.0.0: configure
ghcjs-dom-0.9.0.0: build
ghcjs-dom-0.9.0.0: copy/register
ghcjs-dom-hello-5.0.0.0: configure (lib + exe)
Configuring ghcjs-dom-hello-5.0.0.0...
ghcjs-dom-hello-5.0.0.0: build (lib + exe)
Preprocessing library ghcjs-dom-hello-5.0.0.0...
[1 of 1] Compiling HelloMain        ( src/HelloMain.hs, .stack-work/dist/x86_64-linux/Cabal-1.24.2.0/build/HelloMain.o )
               
/home/roland/Projekte/ghcjs-dom-hello/src/HelloMain.hs:22:5-52: error:
    • No instance for (Language.Javascript.JSaddle.Marshal.String.ToJSString
                         (Maybe [Char]))
        arising from a use of ‘setInnerHTML’
    • In a stmt of a 'do' block:
        setInnerHTML body (Just "<h1>Kia ora (Hi)</h1>")
      In the expression:
        do { doc <- currentDocumentUnchecked;
             body <- getBodyUnsafe doc;
             setInnerHTML body (Just "<h1>Kia ora (Hi)</h1>");
             _ <- on doc click
                  $ do { (x, y) <- mouseClientXY;
                         newParagraph <- uncheckedCastTo HTMLParagraphElement
                                         <$> createElement doc "p";
                         .... };
             .... }
      In an equation for ‘helloMain’:
          helloMain
            = do { doc <- currentDocumentUnchecked;
                   body <- getBodyUnsafe doc;
                   setInnerHTML body (Just "<h1>Kia ora (Hi)</h1>");
                   .... }
Completed 11 action(s).

--  While building package ghcjs-dom-hello-5.0.0.0 using:
      /home/roland/.stack/setup-exe-cache/x86_64-linux/Cabal-simple_mPHDZzAJ_1.24.2.0_ghc-8.0.2 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.24.2.0 build lib:ghcjs-dom-hello exe:ghcjs-dom-hello exe:ghcjs-dom-hello-warp exe:ghcjs-dom-hello-webkitgtk --ghc-options " -ddump-hi -ddump-to-file"
    Process exited with code: ExitFailure 1
roland@goms:~/Projekte/ghcjs-dom-hello$