DIP83
| Title: | Configurable Assert Diagnostics |
|---|---|
| DIP: | 83 |
| Version: | 1 |
| Status: | Draft |
| Created: | 2015-10-01 |
| Last Modified: | 2015-10-1 |
| Author: | Per Nordlöw |
| Links: |
Abstract
Allow for assert's to do pretty printing of its failing expression. Printing is configurable via specific sets of (template) function overloads.
Rationale
Typically added/modified by the developer by changing the flags to calls to rdmd/dub/scons etc upon failure with `-unittest` for a specific failing module. This will be more convenient than explicitly adding the prints of `lhs` and `rhs` in the failing unittest.
Description
This DIP proposes to add library-level configurable diagnostics to failing calls to assert(expr) typically called in unittest-blocks.
This diagnostics is activated only when DMD is called with a specific command line flag, say `-unittest=verbose`.
If DMD is called with this flag it will then rewrite the AST to replace calls to
```D assert(lhs == rhs) ```
with
```D assertBinOp(A_FILE, A_LINE, A_COLUMN, "==")(lhs == rhs, lhs, rhs) ```
where
```D assertBinOp(string file, uint line, uint column, string op)(E, L, R)(lazy E expression, lazy L lhs, lazy R rhs) ``` and similarly for unary expressions.
The default implementation of `assertBinOp` (preferrably defined somewhere in `druntime`) would be some standard D code that mimics the current behaviour of `assert(expression)` by throwing an `AssertExpression` if `cast(bool)expression` is `false` (dont know about the behaviour of nothrow/@nogc discussed above though). The Phobos-developer can then do what he likes with the information he needs in the extra arguments in specific templated overloads of `assertBinOp`.
This specific behaviour could be extendable by adding (typically templated) overloads of `assertBinOp` for specific sets of types (concepts).
Then we could get the extendability we want in testing-frameworks such as this without adding a new `assert`-overload-set and without sacrifycing default memory usage in DMD/Phobos unittests.
Further with this solution we could add cool diagnostics behaviour in `assertBinOp` for failing array/range comparisons aswell in a format. This diagnostics could even have different pretty printing backends such as HTML.
For example a failing
```D assert([1,2,3] == [1,2,4]); ```
could fancy-print
``` ([1,2,3][2] being 3) != ([1,2,4][2] being 4) ```
or for aggregates a failing
```D struct A { int x, y; } auto a = A(1,2); auto b = A(1,3); assert(a == b); ```
could fancy-print
``` (a.y being 2) != (b.y being 3) ```
Thereby giving D yet another feature, I've not seen in any other language :)
If needed I'd be more than happy to help out implementing this solution in dmd/druntime/phobos. I'm guessing that parts of the solution list at
https://issues.dlang.org/show_bug.cgi?id=5547#c3
can be reused, right?
Impact
Copyright
This document has been placed in the Public Domain.
