The "No-Brainer" Tool
The No-Brainer Tool performs a static check for arity errors and unused bindings on the program in the definitions window.
1 Arity Errors
No-Brainer operates on any sequence of top-level definitions, but makes a weak soundness claim ONLY for code contained in top-level module expressions.
No-Brainer makes the following soundness-like claim: if No-Brainer identifies a piece of code as containing an arity error, then that code (if evaluated) will raise an exception[*].
No-Brainer makes no specific completeness-like claim. No-Brainer makes two passes; the first to associate identifiers with arity specifications, the second to identify arity errors. No-Brainer will correctly associate lambda expressions with definitions (both lexical and top-level) when the lambda expression is directly contained by the definition or is the only possible evaluation result of the expression directly contained in the definition. Any mutation of the identifier causes No-Brainer to remove this identifier from its table (and hence to make no claims about its applications). No-Brainer will identify arity errors only in applications where the function position is directly an identifier.
2 Unused Bindings
No-Brainer also reports unused bindings in the user’s program. In particular, No-Brainer reports an error for each let, letrec, or lambda which meets the following criteria:
It binds an identifier which is not referred to anywhere in that identifier’s scope.
The unused identifier has a source position.
The second of these is intended to filter out bindings introduced by macro expansion, as these are less likely to be errors in the user’s program. Note that the position check is applied to the identifier, and not to the let, letrec, or lambda, so that macro-based special forms which bind identifiers specified by the user will be treated like ordinary binding forms.
Since unused bindings are less likely to be errors than arity mismatches, they are reported in a later section of the output. Also, since unused lambda bindings are less likely to be errors than unused let/rec bindings, the unused bindings error messages are separated into these two categories
3 Installing the Tool
No-Brainer is a Planet package. It’s one of those planet packages that adds a tool rather than providing functions, though, so there’s a stub file just so that you can install it using a planet require.
Evaluate this program:
#lang scheme (require (planet clements/no-brainer))
... in the module language, and then restart DrScheme to see the tool’s button.
4 Using the Tool
To use No-Brainer, click the No Brain button. No-Brainer will print a series of errors detected. All output is confined to a window entitled Things you might want to Fix.
The user’s interface to no-brainer is primitive, in that references to source-code positions are accomplished by mzscheme’s native format ability to represent syntax objects. That is, a syntax object which refers to line and line position within the current file will be printed so as to reveal that information.
Example:
... |
bindings (end-selection) (from (#<syntax:2487:22>)) unused in expression #<syntax:2484:14> |
... |
5 Running Time
No-Brainer makes two passes over the code, and visits each expression once during each pass. In theory, it could be quadratic in running time, as it must perform a set-union operation on lists of identifiers at each step, but this should never dominate the running time. No-Brainer is observed to take roughly ten seconds on files of roughly two thousand lines.
Useful? You decide.
[*] Note that the exception may not be an arity error, but instead an undefined/uninitialized error. This can happen because the application precedes the definition, or because the evaluation of the definition raised an exception.