Parsing of semicolon followed by binary operator is confusing #6961
Comments
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Comment author: elnatan I should add: I think the first parsing is the most natural, with the 'let' extending past the semicolon. My preference would be to make the latter parsing be a syntax error---I don't know why a binary operator should be allowed after a semicolon. |
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Comment author: nevor Actually, it's the optional last silent semicolon that is kicking here. Sequences and records can end with a useless semicolon for cosmetics and to ease future code re-organisation : let () = In your case when the parser hits "*", it reduces what it as parsed so far if it's an expression and, thanks to the optional semicolon, "let x = x + 1 in x;" is a valid expression. |
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Comment author: aalekseyev I hit what looks like the same bug. [fun x -> y; >>= z] parses as [(fun x -> y) >>= z], which is very confusing. |
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Comment author: @damiendoligez |
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This issue has been open one year with no activity. Consequently, it is being marked with the "stale" label. What this means is that the issue will be automatically closed in 30 days unless more comments are added or the "stale" label is removed. Comments that provide new information on the issue are especially welcome: is it still reproducible? did it appear in other contexts? how critical is it? etc. |
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This issue has been open one year with no activity. Consequently, it is being marked with the "stale" label. What this means is that the issue will be automatically closed in 30 days unless more comments are added or the "stale" label is removed. Comments that provide new information on the issue are especially welcome: is it still reproducible? did it appear in other contexts? how critical is it? etc. |
Original bug ID: 6961
Reporter: elnatan
Status: acknowledged (set by @damiendoligez on 2015-08-19T15:03:44Z)
Resolution: open
Priority: normal
Severity: minor
Version: 4.02.1
Target version: later
Category: lexing and parsing
Related to: #4627 #5936 #6327 #6828 #7193
Monitored by: @gasche @diml @hcarty
Bug description
The following two definitions are parsed differently. Is there a good justification for this?
let () =
let x =
let x = 5 in
let x = x + 1 in
ignore x; x
in
Format.printf "%d\n" x (* prints 6; the second 'let' extends past the semicolon *)
;;
let () =
let x =
let x = 5 in
let x = x + 1 in
x; * x
in
Format.printf "%d\n" x (* prints 30; the second 'let' ended at the semicolon *)
;;
To make things more confusing, replacing '*' with '+' in the second example causes the parsing to be the same as the 'ignore' case: the program prints 6, with the 'let' extending past the semicolon and '+' being interpreted as a unary operator (and the 'x;' leading to 'Warning 10: this expression should have type unit.').
(This example is somewhat contrived, but something like this is rather realistic when using monadic operators instead of '*' and '+'.)
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