I have a project where I in one module (A) I decided to build something like a minimal framework for the other bigger module (B) to use. One of the main points of the framework is that the monadic stacks are hidden behind `newtype`s and in those monads you can only use the functions that the module A provides. The module A can be found here: https://github.com/mrkkrp/mmark/blob/ghc-bug-newtypes/Text/MMark/Parser/Internal.hs There are two monadic stack wrapped with newtypes: `BParser` and `IParser`. The module B is this: https://github.com/mrkkrp/mmark/blob/ghc-bug-newtypes/Text/MMark/Parser.hs But it's not really relevant. Now if I change `newtype`s to `type` synonyms like so: ``` type BParser a = ParsecT MMarkErr Text (State BlockState) a type IParser a = StateT InlineState (Parsec MMarkErr Text) a ``` and do corresponding minor corrections, I get 2x less allocations and almost 2x faster code (before): ``` Case Allocated GCs Max with file: data/bench-yaml-block.md 119,080 0 11,088 with file: data/bench-thematic-break.md 74,368 0 10,224 with file: data/bench-heading.md 901,928 0 9,432 with file: data/bench-fenced-code-block.md 145,744 0 9,368 with file: data/bench-indented-code-block.md 124,312 0 9,368 with file: data/bench-unordered-list.md 2,010,496 1 10,784 with file: data/bench-ordered-list.md 2,025,016 1 10,728 with file: data/bench-blockquote.md 1,961,672 1 42,648 with file: data/bench-paragraph.md 2,084,104 1 42,648 with file: data/comprehensive.md 25,899,496 24 44,200 benchmarking with file: data/comprehensive.md time 3.590 ms (3.578 ms .. 3.601 ms) 1.000 R² (1.000 R² .. 1.000 R²) mean 3.555 ms (3.546 ms .. 3.565 ms) std dev 31.07 μs (24.63 μs .. 39.90 μs) ``` After: ``` Case Allocated GCs Max with file: data/bench-yaml-block.md 116,864 0 11,088 with file: data/bench-thematic-break.md 64,776 0 10,392 with file: data/bench-heading.md 615,672 0 9,432 with file: data/bench-fenced-code-block.md 144,736 0 9,368 with file: data/bench-indented-code-block.md 123,352 0 9,368 with file: data/bench-unordered-list.md 795,072 0 41,568 with file: data/bench-ordered-list.md 808,808 0 41,512 with file: data/bench-blockquote.md 826,392 0 41,568 with file: data/bench-paragraph.md 881,432 0 41,568 with file: data/comprehensive.md 10,945,104 10 44,440 benchmarking with file: data/comprehensive.md time 2.451 ms (2.448 ms .. 2.456 ms) 1.000 R² (1.000 R² .. 1.000 R²) mean 2.432 ms (2.427 ms .. 2.437 ms) std dev 15.86 μs (13.16 μs .. 19.01 μs) ``` I'm not great at reading non-trivial core, but I gave it a shot and dumped some core. One thing I noticed that core for the module A is the same in both cases. Then the problem is an inter-module problem, probably like when you don't dump definitions into interface files with `INLINEABLE` and end up without specialization, but here it perhaps has to do with the fact that B has no information about internals of the monadic stacks from A, but I'm not sure. Here is the beginnings of core dumps (before): ``` ==================== Tidy Core ==================== 2017-12-23 04:55:17.967944505 UTC Result size of Tidy Core = {terms: 210,169, types: 209,675, coercions: 82,492, joins: 973/3,753} ``` After: ``` ==================== Tidy Core ==================== 2017-12-23 04:58:46.386265108 UTC Result size of Tidy Core = {terms: 301,256, types: 263,104, coercions: 28,560, joins: 1,726/5,393} ``` So it looks like `newtype` wrapping reduces GHC's ability to create join points SPJ talked about recently. I do understand that you expect a minimal reproducing example but I could not produce it even though I spent several hours in futile attempts. I started by creating a small project, defining a similar stack with a `newtype` wrapper and using it in a simple parser in another module. Then I benchmarked the parser. There is no difference between `newtype`d and code and the code that uses just type synonyms. I tried different variations, no difference. The core output is too big for me to analyze, it's like 25 Mb and 33 Mb, and I have no idea what's going on there. You're welcome to experiment with the repo, there are benchmarks for memory usage and criterion benchmarks for speed of execution: https://github.com/mrkkrp/mmark I have created two branches I won't touch, one with newtypes and another one with type synonyms: - https://github.com/mrkkrp/mmark/tree/ghc-bug-newtypes - https://github.com/mrkkrp/mmark/tree/ghc-bug-type-synonyms Just checkout one of these and run `stack bench`. This is the commit that changes newtypes to type synonyms: https://github.com/mrkkrp/mmark/commit/759d8d4aa52dd57a393299c63e8c9b70d0d43290 I'm submitting this because my friend convinced me that it's better to let you know (even though I could not create a minimal reproducing example on my own) than to let it go completely unnoticed.