Re: Why are nested brackets disallowed?

30 Jan 2019


      I put up a patch which removes this restriction (for untyped expressions).

https://gitlab.haskell.org/ghc/ghc/merge_requests/259

On Thu, Jan 24, 2019 at 11:46 AM Matthew Pickering
 wrote:
...
There is a check in `RnSplice` which errors on the following program
with nested brackets.
```
prog = [| [| True |] |]
T.hs:4:11: error:
    • Template Haskell brackets cannot be nested (without intervening splices)
    • In the Template Haskell quotation [| True |]
      In the Template Haskell quotation [| [| True |] |]
  |
4 | prog = [| [| True |] |]
  |           ^^^^^^^^^^
```
As far as I can see the check was added in 2013 in this commit,
https://github.com/ghc/ghc/commit/d0d47ba76f8f0501cf3c4966bc83966ab38cac27#d...
But there is no note, no tests and no comment about why it was added.
I removed the check and added a `BracketE` constructor to the
template-haskell AST and the
code compiles fine.
I can also construct a program which needs to be spliced twice and
this also works fine.
```
func Add = [| (+) |]
func Mul = [| (*) |]
f1 "+" = [| Add |]
f1 "*" = [| Mul |]
comb s = [| func $(f1 s) |]
```
```
res = $($(comb "*"))
```
So it seems the restriction is quite arbitrary but I was wondering if
I was missing some limitation which meant this check was added. I
would not be surprised if something more complicated goes wrong with
splicing.
Cheers,
Matt