More informative errors

[cpsievert]

Not sure how easy/feasible this is, but it would be great if we could probably more feedback if parsing fails.

ggsql parse "SELECT sex, class, COUNT(*) AS total, SUM(survived) AS num_survived FROM titanic WHERE sex IS NOT NULL AND class IS NOT NULL GROUP BY sex, class VISUALISE class AS x, sex AS y, num_survived/total AS fill, total AS size DRAW rect LABEL title => 'Mosaic Plot: % Survived by Sex and Class', x => 'Class', y => 'Sex', fill => 'Survival Rate', size => 'Group Size'"

Parse error: Parse error: Parse tree contains errors

[teunbrand]

I'm hijacking this issue to be more generally about errors that have insufficient information.

This query has no data, but the error raised is about columns not existing. While this is technically correct, the more informative message would let users know they forgot to add data.

VISUALISE bill_len AS x, bill_dep AS y
DRAW point

#> Failed to prepare data: Validation error: Layer 1: aesthetic 'x' references non-existent column 'bill_len'

[georgestagg]

We raise errors in a few places:

• From tree-sitter, the ever useful: "Parse tree contains errors".
• When we are converting tree-sitter's CST into our own Plot specifications.
• When we are writing out with a specific writer, e.g. vegalite.

We have pretty good control over the latter cases, but not the first. @cscheid has an idea and working example of making these errors much better, but it seems a little experimental and I want to hold off trying it here until we've nailed the main syntax. Nevertheless, I would like to try it here.