Hi Tim.
1. You can use unset? to check for words that do not return values .
>> unset? print ""
== true
>> if unset? print "" [result: "<<unset>>"]
2. If you need to assign the return value of the tested function to some
local word, you can use set/any to avoid an error.
3. Why doesn't print return a value? Wel;l, I suspect that is because
the purpose of print is to display its argument. If print were to return
a value, it would not only be displaying its argument, it would force
the REBOL interpreter to display print's return value as well. I.e. you
would see a combination of the printed argument and print's return value
on the screen. Instead of
>> print ""
>>
which is the expected result, for instance, you would get
>> print_with_return: func [arg] [ print arg return true ]
>> print_with_return ""
== true
>>
BUT the argument to print was NOT "==true" it was "".
Hope this helps.
Elan
Tim Johnson wrote:
>Hello Rebols:
>I've got a question:
>I've written a function called 'fetch, which has print
>stubs to help follow the flow control.
>
>If you evaluate 'print with 'fetch, no value is
>returned.
>
>Can someone tell me why this is so?
>TIA
>tim
>
>(example and code follows)
>Example:
>
>
>>>do %test-fetch.r
>>>
>>>
>Script: "Untitled" (none)
>
>
>>>fetch 'abc
>>>
>>>
>type = 'word
>val is unset
>returning unset words as default value
>== none
>
>
>>>fetch 'print
>>>
>>>
>type = 'word
>val is set
>
>The code for 'fetch and helper function 'safe-reduce
>follows:
>REBOL[]
>; -------------------------------------------------------------------------------
>make object! [
> default-value: none
> set 'fetch func[ {safe data retrieval. Handles any value}
> val [any-type!]
> /seed {set default value} /deep {If block, reduce}
> /same {If unset return 'val}
> /local tmp][
> either word? val[
> print "type = 'word"
> either value? val[
> print "val is set"
> get val
> ][
> print "val is unset"
> either same[
> print "returning unset word as the word"
> val
> ][
> print "returning unset words as default value"
> default-value
> ]
> ]
> ][ ; any other type but 'word
> print ["type = " (type? val)]
> either block? val[
> either deep[
> safe-reduce/deep val
> ][
> safe-reduce val
> ]
> ][
> val
> ]
> ]
> ]
> ]
>; -------------------------------------------------------------------------------
>safe-reduce: function[blk[block!] {reduce block, leaving unset values}
> /deep {reduce nested blocks} ][rblk _reduce][
> dummy: func[val /deep][val] ; pass
> ; below a function assignment using the prefix colon
> _reduce: either deep[:safe-reduce][:dummy]
> ; since 'reduce is a native value (compiled into binary)
> ; try it first
> either all[(not deep) (rblk: attempt[reduce blk])][
> print "Used native 'reduce"
> rblk
> ][
> print "Block with unset values, using 'safe-reduce"
> rblk: make block! length? blk
> foreach element blk[
> either word? element[
> either value? element[
> append rblk (get element)
> ][
> append rblk element
> ]
> ][
> either block? element[
> append/only rblk _reduce/deep element
> ][
> append rblk element
> ]
> ]
> ]
> rblk
> ]
> ]
>
>
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