Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Esteemed and hallowed Monks: Trying to force upper case on Moose args taken from command line - eg FX currency codes - e.g. 
$lookup usd eur gbp chf

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Yes - I could put it into the calling code - but how would I get Moose to force it itself - I tried this from Moose docs: 
trigger => sub { $_[0]->code( uc( $_[0]->code ) );

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but I get message that I am unable to to modify read-only value despite Moose docs saying that trigger can modify RO values. What is correct way to force upper case (and thus in general) 'pre-preocess' object constructor arguments? Thank-you.

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Re: pre-preprocess Moose args in constructor
by haj (Vicar) on Apr 14, 2018 at 09:28 UTC
    There are several ways to do this, depending on how much validation you want to add. A lightweight method without any validation is to add a BUILDARGS routine to your class (see Moose::Manual::Construction): 
    sub BUILDARGS {
    my $class = shift;
    my %args = ref $_[0] ?  %{$_[0]} : @_;
    $args{code} = uc $args{code} if exists $args{code};
    return \%args;
}

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    A more idiomatic method is to add a Moose type for the currency code, check for its validity, and then allow lowercase input by converting it under the hood by Moose coercion. Here's a complete example:
    Read more... See the code (896 Bytes)
    BTW: I don't think that triggers are supposed to be allowed to change readonly attributes. Where did you read that?
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