To make a function with default argument, I tried this:
f: function [a b] [either unset? :b [a + 1] [a + b]] f 5 f 3 5 then I receive this message *** Script Error: f is missing its b argument.
So, what shall I do?
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2 Answers
There's a trick to implement variable-arity functions that some of the built-ins use, most notably help:
>> ? help ... ARGUMENTS: 'word [any-type!] Specify your argument as quoted and belonging to any-type! typeset. Or, alternatively, list the allowed types and include unset! in it.
>> foo: func [a 'b [unset! integer!]][a + do pick [1 b] unset? :b] == func [a 'b [unset! integer!]][a + do pick [1 b] unset? :b] >> foo 5 == 6 >> foo 3 5 == 8 This, however, comes at a certain price:
- Optional arguments must come in succession after the mandatory ones.
- All such function calls within a script now need to be wrapped in parentheses, because otherwise there's no way for the interpreter to guess where your variadic expression ends.
- You lose the benefits of type-checking, since
any-type!accepts any argument;'in front of the argument also enforces specific semantics, which makes such variadic functions even more cumbersome. - Your code becomes unidiomatic: the conventional way to address the need for optional arguments is via refinements. Variadic functions are usually implemented with a single
block!argument, which might even be dialected.
Such an approach is justified only for user-facing polymorphic functions, intended to be used from command-line prompt (such as help) or any other kind of interface that provides clear-cut boundaries for typed expressions (e.g. end of the line, special terminating symbol), and even then the number of optional arguments is kept at minimum.
2 Comments
lyl
Very detail description on this item! Thank you! And also very interesting trick --
pick [1 b] unset? :b , I just find that index of pick can be logic!, but where to find how to use it?
9214
@lyl it's a form of the ternary operator. Expression picks 1st or 2nd value depending on its logical value. Same as in
either, except that branches are not evaluated.You can use a refinement. See: http://helpin.red/Functions.html
For example:
>> increase: function [a /add b] [either none? b [a + 1] [a + b]] == func [a /add b][either none? b [a + 1] [a + b]] >> increase 3 == 4 >> increase/add 3 5 == 8 
2 Comments
lyl
Thank you. Yes, refinement is a solution. But I think this deviates from the original idea of default argument in function.
Maciej Łoziński
I'm afraid it's not possible in Red. You can also try to use block as an argument, and block can have as many elements as you want. But that's just a workaround.