No instance for (ToJSVal Value)

[mightybyte]

I recently encountered this error while building jsaddle-0.9.4.0:

[12 of 16] Compiling Language.Javascript.JSaddle.Value ( src/Language/Javascript/JSaddle/Value.hs, dist/build/Language/Javascript/JSaddle/Value.js_o )

src/Language/Javascript/JSaddle/Value.hs:567:15-21: error:
    • No instance for (ToJSVal Value) arising from a use of ‘toJSVal’
    • In the expression: toJSVal
      In an equation for ‘valMakeJSON’: valMakeJSON = toJSVal

Anyone come across this?

[hamishmack]

Where is this instance? It depends.

Compiling with GHC : jsaddle
Compiling with GHCJS (not reflex platform) : ghcjs-base
Compiling with GHCJS (using reflex platform) : patch for aeson

When using the reflex platform, make sure there is no constraint preventing cabal from using the patched patched version of aeson from your ghcjs-pkg list. If it cannot use that version it will download and build aeson from hackage (without the patch needed).