GHCi leaks are not exactly new, but this one is especially easy to trigger/reproduce.

The memory allocated by f never gets freed.

PS E:\binary-perf> ..\ghc-8.6.1\bin\ghci.exe -fno-it
GHCi, version 8.6.1: http://www.haskell.org/ghc/  :? for help
Prelude> f = [1 .. 20000000] :: [Int]
Prelude> length f
20000000
Prelude> f = [1 .. 20000001] :: [Int]
Prelude> length f
20000001
Prelude> f = [1 .. 20000002] :: [Int]
Prelude> length f
20000002
Prelude> f = [1 .. 20000000] :: [Int]
Prelude> length f
20000000
Prelude>

Or using head and even simpler:

PS E:\binary-perf> ..\ghc_commonAsm\inplace\bin\ghci.exe -fno-it
GHCi, version 8.7.20181207: http://www.haskell.org/ghc/  :? for help
Loaded package environment from E:\binary-perf\.ghc.environment.x86_64-mingw32-8.7.20181207
Prelude> f = replicate 2000000 False
Prelude> length f
2000000
Prelude> f = replicate 2000000 False
Prelude> length f
2000000
Prelude> f = replicate 2000000 False
Prelude> length f
2000000
Prelude> f = replicate 2000000 False
Prelude> length f

GHC just keeps using more and more memory when I repeat this.