[Rd] Extending suggestion for stopifnot

[Brian Rowe]

If all you care about is emulating static type checking, then you can also accomplish the same thing with lambda.r using type constraints on function definitions.

e.g.
> f(m) %::% matrix : matrix
> f(m) %as% { m }

> f(as.data.frame(matrix(rnorm(12),nrow=3)))
Error in UseFunction("f", ...) : No valid function for 'f(data.frame)'

> f(1)
Error in UseFunction("f", ...) : No valid function for 'f(1)'

> f
<function>
[[1]]
f(m) %::% matrix:matrix
f(m) %as% …



On Aug 20, 2013, at 4:36 PM, Peter Langfelder <peter.langfelder at gmail.com> wrote:

> On Tue, Aug 20, 2013 at 11:41 AM, ivo welch <ivo.welch at anderson.ucla.edu> wrote:
>> I am using a variant of stopifnot a lot.  can I suggest that base R
>> extends its functionality?  I know how to do this for myself.  this is
>> a suggestion for beginners and students.  I don't think it would break
>> anything.
>> 
>> first, I think it would be more useful if it had an optional character
>> string, so users could write
>> 
>>  stopifnot( is.matrix(m), "m is not a matrix" )
>> 
>> this would mean that stopifnot would have to detect whether the last
>> argument is a string.  (I think stopifnot should have had only one
>> condition, and one should have used all() to test multiple conditions,
>> but this is a bridge that was already crossed.)  upon failure,
>> stopifnot should print the character string.  that's it.
>> 
>> 
>> A second enhancement would be a "smart string", which knows that
>> everything inside {{...}} should be evaluated.
>> 
>>  stopifnot( is.matrix(m), "m is not a matrix, but a {{class(m)}}" )
> 
> I think using a function (in this case paste) is cleaner:
> 
> paste("m is not a matrix, but a", class(m))
> 
> It avoids adding a new convention ("evaluate everything between {{
> }}") and has additional arguments.
> 
>> 
>> 
>> my own programming variant looks even nicer,
>> 
>>   is.matrix(m) %or% "m is not a matrix but a {{class(m)}}"
> 
> In R you can write it as
> 
> is.matrix(m) || stop("m is not a matrix but a ", class(m))
> 
> Examples:
> 
> m = 1
>> is.matrix(m) || stop("m is not a matrix but a ", class(m))
> Error: m is not a matrix but a numeric
> 
>> m = matrix(0,2,2)
>> is.matrix(m) || stop("m is not a matrix but a ", class(m))
> [1] TRUE
> 
> But the construct
> 
> if (!is.matrix(m)) stop("m is not a matrix but a ", class(m))
> 
> is more readable for people not used to Pearl.
> 
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