# Faking named parameters and enforcing required params

**URL:** <https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272>\
**Category:** Ruby\
**Created:** [June 24, 2008, 8:14am UTC](https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272 "2008-06-24T08:14:20Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [June 24, 2008, 8:14am UTC](https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272/1 "2008-06-24T08:14:20Z")

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I come from years of using a language with named params. I prefer the  
explicitness of it, so I’m tending to do a lot of it in my Ruby code  
using a hash to fake it

object.method(:shape=\>‘square’, :size=‘medium’)

What I am wondering is whether there’s an idiomatc preference to dealing  
with the hash inside the method?

Currently, I just do the prozaic:

def task(parameters)  
shape = parameters[:shape]  
size = parameters[:size]

Should I be doing something else?

In that other language (Lasso), it also offered a way to declare  
required and optional parameters

define\_tag:‘task’,  
-required = ‘shape’,  
-optional = ‘size’;

```
...blah...

```

/define\_tag;

which then got used like this:

object-\>task(-shape=‘square’, -size=‘large’) # legal  
object-\>task(-shape=‘square’) # legal  
object-\>task(-size=‘medium’) # failed for lack of required param

The required and optional features combined with some other stuff got  
very close to a full design by contract implementation.

Anyway, I figured out I could simulate the assignment side of required  
and optional with this:

def task(parameters)  
@shape = parameters[:shape]  
@size = parameters[:size] ||= ‘medium’

What I have’t figured out is an elegant pattern in Ruby for enforcing  
required parameters – other than a bunch of hand written exception  
coding.

To summarize my questions:

A) is there something better than x = parameters[:x]?  
B) if :x isn’t in (parameters), what’s the best way to complain to the  
developer about that?

Many thanks.

– gw

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<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [June 24, 2008, 10:20am UTC](https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272/2 "2008-06-24T10:20:00Z")

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On Tue, Jun 24, 2008 at 8:12 AM, Greg W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:  
Maybe something like

class Hash  
def require\_all\_keys \*keys  
keys.all?{ |k| has\_key? k } or raise ArgumentError # better  
doing an each here and raise an exception which argument was missing  
of course, but I guess you got the idea;)  
end  
def provide\_defaults defaults={}  
defaults.each\_pair{ |nk,nv|  
self[nk]=nv unless has\_key? nk  
}  
end  
end

def whatever params={}  
params.require :size, :age  
params.provide\_defaults :language =\> “en”, :id =\> 42

end

HTH

Robert

–  
[http://ruby-smalltalk.blogspot.com/](http://ruby-smalltalk.blogspot.com/)

* * *

## Les mêmes questions qu’on se pose On part vers où et vers qui Et comme indice pas grand-chose Des roses et des orties.

Francis Cabrel

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<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [June 24, 2008, 10:20am UTC](https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272/3 "2008-06-24T10:20:35Z")

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BTW I guess Facets has things like these already!

Robert

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**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [July 6, 2008, 7:07am UTC](https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272/4 "2008-07-06T07:07:40Z")

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Greg W. wrote:

> I come from years of using a language with named params. I prefer the  
> explicitness of it, so I’m tending to do a lot of it in my Ruby code  
> using a hash to fake it
> 
> object.method(:shape=\>‘square’, :size=‘medium’)
> 
> What I am wondering is whether there’s an idiomatc preference to dealing  
> with the hash inside the method?  
> object-\>task(-shape=‘square’, -size=‘large’) # legal  
> object-\>task(-shape=‘square’) # legal  
> object-\>task(-size=‘medium’) # failed for lack of required param
> 
> The required and optional features combined with some other stuff got  
> very close to a full design by contract implementation.

Interestingly, I have also been working on something very similar to  
this recently. [a named parameter parser for ruby]

Basically it parses an array as a parameter list, with some required and  
some optional parameters.  
ex:

[first remember that you can pass parameters to a method via \* for a  
variable number of parameters, i.e. def method \*args; end passes in args  
as an array of all parameters]

the syntax it uses is:

def method1 \*as  
required1, required2, optional1 = as.args [:required1, :required2],  
{:optional1 =\> 3}  
end

and you call it either named or unnamedly, a la:  
method1 4, 5, 6  
or  
method1 4, 5  
or  
method1 :required1 =\> 1, :required2 =\> 2  
or  
method1 :required1 =\> 1, :required2 =\> 2, :optional1 =\> 4  
or  
method1 1, 2, :optional1 =\> 4  
whichever floats your boat.

Try it out, let me know if it works.

the file is

[http://code.google.com/p/ruby-roger-useful-functions/source/browse/trunk/arg\_parser/enhanced\_arg\_parser.rb](http://code.google.com/p/ruby-roger-useful-functions/source/browse/trunk/arg_parser/enhanced_arg_parser.rb)

and docs for it [http://wilkboardonline.com/roger/arg\_parser/doc/](http://wilkboardonline.com/roger/arg_parser/doc/) [see  
args method for description].  
Thanks!

-R

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<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [July 9, 2008, 7:00am UTC](https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272/5 "2008-07-09T07:00:32Z")

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> My question is does anybody know the C code for ‘adding’ a variable to  
> the scope above the current?  
> something like
> 
> scope-\>prev-\>local\_vars.push(name, object);
> 
> Thanks!  
> -R

It appears impossible [from casual glancing at the code] to add an  
arbitrary local variable during runtime. This because it uses a ‘parse  
then run’ style execution.  
I.e.  
a = 3  
b  
is first translated into  
ASSIGN(a, 3)  
CALL(b)

and then run consecutively.  
And  
a = 2  
a  
is translated into  
ASSIGN(a, 2)  
VAR\_VALUE(a) since a is already defined. Then run.

So it appears that currently the only way to arbitrarily add a local  
variable would be to hack the parse tree itself.

Guess I’ll file a requested feature of binding.set\_local\_variable 🙂  
-R

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<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [July 9, 2008, 12:15am UTC](https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272/6 "2008-07-09T00:15:44Z")

</div>

> Basically it parses an array as a parameter list, with some required and  
> some optional parameters.  
> ex:
> 
> [first remember that you can pass parameters to a method via \* for a  
> variable number of parameters, i.e. def method \*args; end passes in args  
> as an array of all parameters]

My question is does anybody know the C code for ‘adding’ a variable to  
the scope above the current?  
something like

scope-\>prev-\>local\_vars.push(name, object);

Thanks!  
-R

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<div class="post-metadata">

**Author:** ![S-gw](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/ed655f/32.png) [@S-gw](https://www.ruby-forum.com/u/S-gw)\
**Post date:** [July 10, 2008, 1:48am UTC](https://www.ruby-forum.com/t/faking-named-parameters-and-enforcing-required-params/141272/7 "2008-07-10T01:48:57Z")

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> So it appears that currently the only way to arbitrarily add a local  
> variable would be to hack the parse tree itself.

Appears that ParseTree and ruby2ruby provide this type of functionality  
[see [Not going dark](http://weblog.raganwald.com/2008/06/not-going-dark.html) for an  
example].

So my question is  
say I’ve got a method  
def method1(a, b=3)  
…  
end  
that I want to allow to accept named parameter [a la method1(2, :b =\> 3)  
or method1(:a =\> 2]

I could either rewrite the original method’s parse tree to allow for  
named parameters [i.e. convert it to]  
def method1(\*args)  
a, b = args.parse [:a], {:b =\> 3}  
…  
end  
then ‘eval’ the newly created function, replacing the original with the  
new

or I could just write a caller function, a la  
alias :original\_method1 :method1

def method1(\*args)  
a, b = args.parse [:a], {:b =\> 3}  
original\_method1(a, b)  
end

so alias chain kind of.  
Any thoughts on which of these would might be preferable?  
Thanks!  
-R