Missing an empty surround operator? #5
Description
@olivier-martinot and myself are writing a pretty-printer for MidML. I got stuck on how to properly pretty-print let ... in, and I think that my final iteration suggests that a new combinator could be useful, but I don't know how to name it.
Specific question
prefix n b empty doc has a short form, jump n b.
What is the short form of surround n b empty doc empty?
Longer story
We want to pretty-print let <lhs> = <rhs> in <body>. All of the subexpressions could be long. The printing discipline we want is as follows:
- if
<lhs>or<rhs>are non-flat, they should be 2-indented on their own lines. (bodyis not indented) - otherwise pack things when they fit
For example we want to allow all of the following:
let <lhs> = <rhs> in <body>
let
<lhs>
= <rhs> in <body>
let <lhs> =
<rhs>
in <body>
let <lhs> = <rhs> in
<body>(Okay, the second example looks very ugly, but (1) this regular structure makes my story applicable to other constructs that contain a mix of keywords and symbols beyond the simple prefix/surround scenarios, and (2) it never occurs in practice, and actually the fact that we never see it probably explains why we find it ugly. I don't want to make the rules or code more complex to forbid it.)
After trying many things, I ended up with the following approach where the code looks simple enough
let print_let_in lhs rhs body =
string "let"
^^ surround 2 1 empty lhs empty
^^ string "="
^^ surround 2 1 empty rhs empty
^^ string "in"
^^ prefix 0 1 empty bodyThe prefix 0 1 empty body form can be simplified into jump 0 1 body.
For lhs and rhs, I would need a version of jump that adds a break on both sides of the main document, not just on the left. Could we have this? What would be a good name for it?
(I don't understand what jump is trying to evoke as a name, how it was chosen.)
Maybe an option would be to add a new parameter to jump for a break on the right: jump n b doc would become jump n b_left doc b_right. The old jump n b doc can be expressed with jump n b doc 0, and my use-case becomes jump 2 1 doc 1.