[racket-dev] very unhelpful beginner language error message, possible fix proposed

From: David Van Horn (dvanhorn at ccs.neu.edu)
Date: Mon Feb 13 17:07:23 EST 2012

On 2/13/12 4:20 PM, Stephen Bloch wrote:
> On Feb 13, 2012, at 8:28 AM, David Van Horn wrote:
>> In BSL, you can detect when the first element of a clause is a variable bound to a function, but I don't follow the reasoning about ISL.  You can't distinguish good from bad uses without running the code because you can't tell if a name refers to a function or a non-function.
> Not reliably, because the student COULD be using a parameter or a local variable in that position.  But in the COMMON case of this error, the student will use a predefined function or an explicitly user-defined top-level function, and it should be possible to recognize those at syntax-check-time.

But even if the name is a parameter, it cannot be bound to a function. 
If it's a local, it either is or isn't a function -- you can tell from 
the definition.

I think it's correct to consider this a syntax error, not a run-time 
error.  It should just have a better message.

> Which leaves
>> cond: question result is not true or false: (lambda (a1) ...)
> as a good error message to report at run time in the rare cases that don't match the above description.

Except you don't want to say lambda in BSL.


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