Install Haskell version 8.2.2: https://www.haskell.org/platform/prior.html
Elm will NOT build in 8.4.2.
Do once, after installing Haskell:
cabal update
Download the compiler source:
git clone git@github.com:elm/compiler.git
Build:
cd .../compiler
cabal sandbox init
cabal install
After a while, the executable will be in .cabal-sandbox/bin/elm
To remove all the build artifacts for another clean build:
cabal clean
rm -r .cabal-sandbox
Unfortunately, building with profiling as documented below doesn't work, since the distributed Haskell libraries don't have their profiling versions. Some magic with stack, to completely rebuild the Haskell libraries, will probably fix it, but I haven't figured that out yet.
To build with profiling, which enables stack backtraces on errors, add the --enable-profiling flag:
cabal install --enable-profiling
To compile Elm code with full stack backtraces on errors:
.../compiler/.cabal-sandbox/bin/elm +RTS -xc make ...