[Haskell-cafe] More liberal than liberal type synonyms

[Dan Doel]

Greetings,

In the process of working on a Haskell-alike language recently, Ed
Kmett and I realized that we had (without really thinking about it)
implemented type synonyms that are a bit more liberal than GHC's. With
LiberalTypeSynonyms enabled, GHC allows:

    type Foo a b = b -> a
    type Bar f = f String Int

    baz :: Bar Foo
    baz = show

because Bar expands to saturate Foo. However, we had also implemented
the following, which fails in GHC:

    type Foo a b = b -> a
    type Bar f = f (Foo Int) (Foo Int)
    type Baz f g = f Int -> g Int

    quux :: Bar Baz
    quux = id

That is: type synonyms are allowed to be partially applied within
other type synonyms, as long as similar transitive saturation
guarantees are met during their use.

I don't know how useful it is, but I was curious if anyone can see
anything wrong with allowing this (it seems okay to me after a little
thought), and thought I'd float the idea out to the GHC developers, in
case they're interested in picking it up.

-- Dan