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Add changelog entry
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‎changelog.d/pr-10644

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---
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synopsis: Show source of project parse error or warning
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packages: [cabal-install]
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prs: 10644
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issues: 10635
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---
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Improves warning and error messages shown when parsing project files and their
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imports.
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## Warning Messages
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To trigger these warning messages, the examples use badly formed comments that
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have a single dash instead of two as is required of a line comment in `.cabal`
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and `.project` files (and imported `.config` files).
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* Before the fix:
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The `cabal.project` file name is repeated. Warnings are misattributed to
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having been in the project rather than from a configuration file imported by
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the project. Warnings are shown in reverse line number order.
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```
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$ ~/.ghcup/bin/cabal-3.12.1.0 build all --dry-run
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...
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Warning:
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/.../ParseWarningProvenance/cabal.project,
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cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
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section '-' on line 123
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/.../ParseWarningProvenance/cabal.project,
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cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
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section '-' on line 3
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/.../ParseWarningProvenance/cabal.project,
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cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
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section '-' on line 2
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/.../ParseWarningProvenance/cabal.project,
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cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
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section '-' on line 1
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/.../ParseWarningProvenance/cabal.project,
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cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
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section '-' on line 123
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/.../ParseWarningProvenance/cabal.project,
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cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
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section '-' on line 3
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/.../ParseWarningProvenance/cabal.project,
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cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
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section '-' on line 2
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/.../ParseWarningProvenance/cabal.project,
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cabal.project, cabal.project, cabal.project, cabal.project: Unrecognized
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section '-' on line 1
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```
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* After the fix:
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The warnings are shown in a list. For warnings within the same `.project` or
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imported `.config` file, warnings are sorted by line number. The file that
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is the source of the warning is shown.
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```
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$ cabal build all --dry-run
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...
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Warnings found while parsing the project file, cabal.project:
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- dir-x/a.config: Unrecognized section '-' on line 1
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- dir-x/a.config: Unrecognized section '-' on line 2
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- dir-x/a.config: Unrecognized section '-' on line 3
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- dir-y/a.config: Unrecognized section '-' on line 123
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- x.config: Unrecognized section '-' on line 1
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- x.config: Unrecognized section '-' on line 2
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- x.config: Unrecognized section '-' on line 3
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- y.config: Unrecognized section '-' on line 123
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```
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## Error Messages from Project
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To trigger these error messages, the examples use badly formed conditions:
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```
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$ cat cabal.project
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-- The following failing condition is not on the first line so we can check the
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-- line number:
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if _
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```
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* Before the fix:
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The parse error is shown with hard line breaks.
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```
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$ ~/.ghcup/bin/cabal-3.12.1.0 build all --dry-run
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...
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Error: [Cabal-7090]
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Error parsing project file /.../ParseErrorProvenance/cabal.project:3:
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"<condition>" (line 1, column 1):
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unexpected SecArgName (Position 1 4) "_"
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```
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* After the fix:
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The snippet that failed to parse may be shown and the parse error is shown
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as one line, with no hard line breaks.
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```
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$ cabal build all --dry-run
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...
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Error: [Cabal-7090]
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Error parsing project file cabal.project:3:
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- Failed to parse 'if(_)' with error:
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"<condition>" (line 1, column 1): unexpected SecArgName (Position 1 4) "_"
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```
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## Error Messages from Imported Config
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With the same setup but now with the error in an imported file:
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```
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$ cat elif.project
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import: dir-elif/elif.config
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$ cat dir-elif/elif.config
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-- The following failing condition is not on the first line so we can check the
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-- line number:
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if false
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elif _
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```
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* Before the fix:
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The project rather than the imported configuration file is shown as the source file.
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```
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$ ~/.ghcup/bin/cabal-3.12.1.0 build all --dry-run
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...
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Error: [Cabal-7090]
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Error parsing project file /.../ParseErrorProvenance/elif.project:4:
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"<condition>" (line 1, column 1):
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unexpected SecArgName (Position 1 6) "_"
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```
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* After the fix:
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The imported configuration file is shown as the source with a snippet of the error.
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```
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$ cabal build all --dry-run
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...
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Error: [Cabal-7090]
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Error parsing project file dir-elif/elif.config:4:
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- dir-elif/elif.config
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imported by: elif.project
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- Failed to parse 'elif(_)' with error:
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"<condition>" (line 1, column 1): unexpected SecArgName (Position 1 6) "_"
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```

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