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23 lines
2.2 KiB
Markdown
23 lines
2.2 KiB
Markdown
# DSL reference: errors and transparency
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As soon as you have a programming language, you start having the problem *What is my code doing, and why?* This includes getting syntax errors -- which are always annoying -- as well as the even more annoying problem of a program which parses without syntax error but doesn't do what you expect.
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The `syntax error` message is cryptic: it says `syntax error at ` followed by the next symbol it couldn't parse. This is good, but (as of 5.0.0) it doesn't say things like `syntax error at line 17, character 22`. Here are some common causes of syntax errors:
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* Don't forget `;` at end of line, before another statement on the next line.
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* Miller's DSL lacks the `++` and `--` operators.
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* Curly braces are required for the bodies of `if`/`while`/`for` blocks, even when the body is a single statement.
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As for transparency:
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* As in any language, you can do `print`, or `eprint` to print to stderr. See [Print statements](reference-dsl-output-statements.md#print-statements); see also [Dump statements](reference-dsl-output-statements.md#dump-statements) and [Emit statements](reference-dsl-output-statements.md#emit-statements).
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* The `-v` option to `mlr put` and `mlr filter` prints abstract syntax trees for your code. While not all details here will be of interest to everyone, certainly this makes questions such as operator precedence completely unambiguous.
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* The `-T` option prints a trace of each statement executed.
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* The `-t` and `-a` options show low-level details for the parsing process and for stack-variable-index allocation, respectively. These will likely be of interest to people who enjoy compilers, and probably less useful for a more general audience.
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* Please see [type-checking](reference-dsl-variables.md#type-checking) for type declarations and type-assertions you can use to make sure expressions and the data flowing them are evaluating as you expect. I made them optional because one of Miller's important use-cases is being able to say simple things like `mlr put '$y = $x + 1' myfile.dat` with a minimum of punctuational bric-a-brac -- but for programs over a few lines I generally find that the more type-specification, the better.
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