# Can anyone reproduce this in menhir?
**URL:** https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627
**Category:** Learning
**Created:** [May 9, 2025, 12:32am UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627 "2025-05-09T00:32:28Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@HumamAlhusaini](https://discuss.ocaml.org/u/HumamAlhusaini)
#### Post date: [May 9, 2025, 12:32am UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/1 "2025-05-09T00:32:28Z")
If I want to parse parameters, something like this:
`(Tralalero, Tralala, Crocodilo, Bombardilo)`
I’d make this menhir rule.
```auto
params:
LPAREN separated_list(COMMA, identifier) RPAREN {}
```
This works!
However, for whatever reason, I want the user to be obligated to add a comma at the end. Like this:
`(Tralalero, Tralala, Crocodilo, Bombardilo,)`
The natural thing is to write something like
```auto
params:
LPAREN separated_list(COMMA, identifier) COMMA RPAREN {}
```
However, this doesn’t work, for whatever reason.
So I attempted something like
```auto
parameters:
LPAREN params RPAREN {}
params:
identifier COMMA params
identifier COMMA
```
Yet this did not work either. Does anybody have an idea on how to do this?
---
### Author:  [@brandon](https://discuss.ocaml.org/u/brandon)
#### Post date: [May 9, 2025, 12:52am UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/2 "2025-05-09T00:52:37Z")
I found this to work:
```auto
params:
LPAREN nonempty_list(terminated(identifier, COMMA)) RPAREN {}
```
---
### Author:  [@HumamAlhusaini](https://discuss.ocaml.org/u/HumamAlhusaini)
#### Post date: [May 9, 2025, 2:09am UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/3 "2025-05-09T02:09:39Z")
Thanks. I love you 💓
---
### Author:  [@HumamAlhusaini](https://discuss.ocaml.org/u/HumamAlhusaini)
#### Post date: [May 9, 2025, 2:24pm UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/4 "2025-05-09T14:24:56Z")
What if I wanted to make it so that the end comma is optional? I attempted something like this
```auto
params:
LPAREN nonempty_list(terminated(identifier, COMMA)) option(COMMA) RPAREN {}
```
but that gave me a shift reduce conflict
```auto
** In state 44, looking ahead at COMMA, reducing production
** separated_nonempty_list(COMMA,use_tree) -> use_tree
** is permitted because of the following sub-derivation:
simple_path_special LBRACE separated_nonempty_list(COMMA,use_tree) option(COMMA) RBRACE // lookahead token appears because option(COMMA) can begin with COMMA
use_tree .
** In state 44, looking ahead at COMMA, shifting is permitted
** because of the following sub-derivation:
simple_path_special LBRACE separated_nonempty_list(COMMA,use_tree) option(COMMA) RBRACE
use_tree . COMMA separated_nonempty_list(COMMA,use_tree)
```
use\_tree is interchangeable with param
---
### Author:  [@nojb](https://discuss.ocaml.org/u/nojb)
#### Post date: [May 9, 2025, 3:02pm UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/5 "2025-05-09T15:02:51Z")
> [@HumamAlhusaini](#):
>
> What if I wanted to make it so that the end comma is optional? I attempted something like this
>
> ```auto
> params:
> LPAREN nonempty_list(terminated(identifier, COMMA)) option(COMMA) RPAREN {}
>
> ```
The problem is that the parser cannot decide upon seeing the last `COMMA` if it is the comma following an identifier as part of the list or if it is the optional comma. You should try instead with
```auto
params:
LPAREN separated_nonempty_list(COMMA, identifier) option(COMMA) RPAREN {}
```
Cheers,
Nicolas
---
### Author:  [@grayswandyr](https://discuss.ocaml.org/u/grayswandyr)
#### Post date: [May 9, 2025, 3:12pm UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/6 "2025-05-09T15:12:11Z")
You might be interested in this [old post](https://gallium.inria.fr/blog/lr-lists/) by @fpottier.
---
### Author:  [@HumamAlhusaini](https://discuss.ocaml.org/u/HumamAlhusaini)
#### Post date: [May 9, 2025, 3:22pm UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/7 "2025-05-09T15:22:10Z")
This gives a shift/reduce error.
```auto
LBRACE;
trees = separated_nonempty_list(COMMA, use_tree);
option(COMMA);
RBRACE
```
```auto
** Conflict (shift/reduce) in state 44.
** Token involved: COMMA
** This state is reached from program after reading:
outer_attrs USE simple_path_special LBRACE use_tree
** The derivations that appear below have the following common factor:
** (The question mark symbol (?) represents the spot where the derivations begin to differ.)
program
items EOF
item items
outer_attrs vis_item
use_declaration
USE use_tree SEMI
(?)
** In state 44, looking ahead at COMMA, reducing production
** separated_nonempty_list(COMMA,use_tree) -> use_tree
** is permitted because of the following sub-derivation:
simple_path_special LBRACE separated_nonempty_list(COMMA,use_tree) option(COMMA) RBRACE // lookahead token appears because option(COMMA) can begin with COMMA
use_tree .
** In state 44, looking ahead at COMMA, shifting is permitted
** because of the following sub-derivation:
simple_path_special LBRACE separated_nonempty_list(COMMA,use_tree) option(COMMA) RBRACE
use_tree . COMMA separated_nonempty_list(COMMA,use_tree)
```
@grayswandyr gave me the idea of using left recursion, so the first thing that I check is that there is a comma at the end.
```auto
use_trees:
| use_tree_list option(COMMA) { $1 }
use_tree_list:
| use_tree { [$1] }
| use_tree_list COMMA use_tree { $3 :: $1 }
```
This worked!
---
### Author:  [@grayswandyr](https://discuss.ocaml.org/u/grayswandyr)
#### Post date: [May 9, 2025, 3:56pm UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/8 "2025-05-09T15:56:57Z")
Just keep in mind that with this technique, the computed list is in reverse order. Depending on your application, you may want to massage it through `List.rev`.
---
### Author:  [@HumamAlhusaini](https://discuss.ocaml.org/u/HumamAlhusaini)
#### Post date: [May 9, 2025, 11:01pm UTC](https://discuss.ocaml.org/t/can-anyone-reproduce-this-in-menhir/16627/9 "2025-05-09T23:01:54Z")
Thanks for reminding me