miller/docs6/docs/reference-dsl-errors.md.in

# DSL errors and transparency

As soon as you have a [programming language](programming-language.md), 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.

The syntax-error message gives you line/column position for the syntax that couldn't be parsed. The cause may be clear from that information, or perhaps not.  Here are some common causes of syntax errors:

* Don't forget `;` at end of line, before another statement on the next line.

* Miller's DSL lacks the `++` and `--` operators.

* Curly braces are required for the bodies of `if`/`while`/`for` blocks, even when the body is a single statement.

As for transparency:

* 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).

* 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.

* 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 long, I generally find that the more type-specification, the better.