I've been trying to check strictness of my model using ppHeapTree function from (slightly altered) ghc-heap-view package and discovered that there seem to be some thunks in HashMap following a series of insert using foldl'. However if I union it with singleton it comes out without thunks.

I've create a little project to demonstrate this behaviour: https://github.com/mantasg/hashmap-test. The version of ghc-heap-view I used doesn't work with GHC 8.6 therefore I used stack lts-11.20 with GHC 8.2. Running it requires stack and can be invoked by:

cd hashmap-test && stack build hashmap-test && stack exec hashmap-test 

HeapTree for test1 = foldl' (\m v -> HM.insert v v m) HM.empty [0..5]

HeapTree for test1 "BitmapIndexed ([ (_thunk (I# 0) (I# 0) 0), (_thunk (I# 1) (I# 1) 1), (Leaf (I# 2) (I# 2) 2), (Leaf (I# 3) (I# 3) 3), (Leaf (I# 4) (I# 4) 4), (Leaf (I# 5) (I# 5) 5) ]) 63" 

HeapTree for test2 = foldl' (\m v -> HM.union (HM.singleton v v) m) HM.empty [0..5]

HeapTree for test2 "BitmapIndexed ([ (Leaf (I# 0) (I# 0) 0), (Leaf (I# 1) (I# 1) 1), (Leaf (I# 2) (I# 2) 2), (Leaf (I# 3) (I# 3) 3), (Leaf (I# 4) (I# 4) 4), (Leaf (I# 5) (I# 5) 5) ]) 63"