[Haskell-cafe] when does ghc(i) expand type synonyms?

[Olaf Klinke]

> ghci> :set +t
> 
> ghci> "foo"
> "foo"
> it :: String
> 
> ghci> reverse "foo"
> "oof"
> it :: [Char]

I'm not a GHC expert, but my guess is:
It must have something to do with unifying the argument types with the
type signature of the function.
In (<>), the two argument and the result types are identical. Hence it
can be safely said that 
    x <> whatever
as long as this expression is valid, has the same type as x. In
contrast, the result type of (++) is always of the shape [a]. Hence
String gets unified with [a] which forces expansion of the type
synonym. Consider:
    class Foo a where {rev :: a -> a; conc :: a -> a -> a}
    instance Foo [a] where {rev = reverse; conc = (++)}
    revconc :: Foo a => a -> a; revconc a = conc a (rev a)
    :t revconc ("foo" :: String)
    revconc ("foo" :: String) :: String
So as long has you hide (++) in a type signature that proves input and
output are the same, synonyms don't get expanded. 

Olaf