# Testing with ppx\_expect and ppx\_deriving.show
**URL:** https://discuss.ocaml.org/t/testing-with-ppx-expect-and-ppx-deriving-show/2040
**Category:** Learning
**Tags:** testing, format
**Created:** [May 23, 2018, 8:07am UTC](https://discuss.ocaml.org/t/testing-with-ppx-expect-and-ppx-deriving-show/2040 "2018-05-23T08:07:58Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@zindel](https://discuss.ocaml.org/u/zindel)
#### Post date: [May 23, 2018, 8:07am UTC](https://discuss.ocaml.org/t/testing-with-ppx-expect-and-ppx-deriving-show/2040/1 "2018-05-23T08:07:58Z")
Hi,
Recently I’ve started to use `ppx_expect` accompanied by `ppx_deriving` (show plugin) for writing unit tests. Alongside with the latest [test integration](http://jbuilder.readthedocs.io/en/latest/tests.html) in `dune` this creates the great workflow, where I can easily review diffs and update tests.
There was one problem I encountered though, which I may need some help with. The function I’m testing returns the data structure which includes absolute file paths. Here is an example:
```ocaml
type resolved = (Fastpack.Module.location * string list) [@@deriving show]
let%expect_test "." =
resolve ".";
[%expect_exact {|
((File {
filename = (Some "/Users/zindel/ocaml/fastpack/test/resolve/index.js");
preprocessors = []
}),
[])
|}]
```
In order to make the test reproducible on other environments where the root path may be different , I am collecting the `show_resolved` output and replacing project root with some constant string in it. The code above becomes:
```ocaml
type resolved = (Fastpack.Module.location * string list) [@@deriving show]
let%expect_test "." =
resolve ".";
[%expect_exact {|
((File {
filename = (Some "/.../test/resolve/index.js");
preprocessors = []
}),
[])
|}]
```
But then, I’ve hit another problem, which is the fact that `ppx_deriving.show` uses the [Format](https://caml.inria.fr/pub/docs/manual-ocaml/libref/Format.html) module and its output is highly dependent on the formatter’s `margin`. Obviously, it would work well if values in the data structure were constants known in advance, but in my case those are absolute paths which depend on the environment. So, what I ended up doing is the set of nasty hacks:
```ocaml
let show resolved =
(* this is to make sure that pp_resolved will produce 1-liner*)
Format.(pp_set_margin str_formatter 100000);
pp_resolved Format.str_formatter resolved;
(* make it more friendly for diffing *)
Format.flush_str_formatter ()
|> String.replace ~sub:"{ " ~by:"{\n "
|> String.replace ~sub:"; " ~by:";\n "
|> String.replace ~sub:" }), " ~by:"\n}),\n"
|> String.replace ~sub:"[\"" ~by:"[\n \""
|> String.replace ~sub:"\"]" ~by:"\"\n]"
```
So, the question is: Is there idiomatic (or, at least, less hack-ish) way to make `show_*/pp_*` produce pretty-printed values which disregard the formatter’s `margin`? Or, maybe, it should be some specific formatter adapted to this use-case? I see the analogy with dumping JSON in JS/Python. It is, obviously, simpler than OCaml type-system, but still. If you specify `indent`, it will return the pretty-printed string. Would something similar be possible with `ppx_deriving.show`?
---
### Author:  [@Yaron\_Minsky](https://discuss.ocaml.org/u/Yaron_Minsky)
#### Post date: [May 24, 2018, 7:48am UTC](https://discuss.ocaml.org/t/testing-with-ppx-expect-and-ppx-deriving-show/2040/2 "2018-05-24T07:48:18Z")
To make the expect test idiom work well, you do indeed need to transform the output to drop information that’s not a deterministic function of the code being run. It sounds like you’ve done some of that already, which seems great.
My recommendation is that you do that transformation before calling the formatter. I’m not sure how easy that is to do in your context, but one way of approaching it would be to use s-expressions for serialization. You could then transform the s-expression before pretty-printing, which should solve the issue.
I also recommend you take a look at Expect\_test\_helpers\_kernel, which provides many useful utilities, including a good s-expression pretty printer, that you can access by calling print\_s.
---
### Author:  [@zindel](https://discuss.ocaml.org/u/zindel)
#### Post date: [May 24, 2018, 11:01am UTC](https://discuss.ocaml.org/t/testing-with-ppx-expect-and-ppx-deriving-show/2040/3 "2018-05-24T11:01:54Z")
Thanks for the suggestion! It looks like `Expect_test_helpers_kernel.print_s` may do what I’m looking for.