Using a lambda as a default in Hash#fetch ruby

Question

I was reading through confident ruby and I was trying out how to define a reusable proc. From the examples given, I wrote this:

DEFAULT_BLOCK = -> { 'block executed' }

answers = {}

answers.fetch(:x, &DEFAULT_BLOCK)

I was expecting it to return block executed since x is not found in the Hash but instead it returned wrong number of arguments (given 1, expected 0) (ArgumentError). What could the problem be? I haven't given the block an argument.

score 9 · Accepted Answer · edited Feb 06 '19 at 09:06

You have, you just don't see it:

WHAT_AM_I_PASSING = ->(var) { var.inspect }

answers = {}

answers.fetch(:x, &WHAT_AM_I_PASSING)
# => ":x"

The block of Hash#fetch provides an argument, the key that you haven't found. You can either accept an argument in your lambda and just ignore it, or make it a proc:

DEFAULT_BLOCK = proc { 'block executed' }
answers.fetch(:x, &DEFAULT_BLOCK)
# => "block executed"

The reason that a proc works, is that lambdas verify that the correct number of arguments were provided while procs don't. The fetch method is calling the proc/lambda with one argument (the key).

score 3 · Answer 2 · answered Feb 06 '19 at 09:01

3

When Hash#fetch takes a block, the key is passed to the block. But your block created from a proc does not take any block argument. Change the definition to:

DEFAULT_BLOCK = -> x { 'block executed' }

answered Feb 06 '19 at 09:01

sawa

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2017kamb · Answer 3 · 2019-02-06T10:37:01.867

    2.6.1 :014 > DEFAULT_BLOCK = -> { 'block executed' }
     => #<Proc:0x00005586f6ef9e58@(irb):14 (lambda)> 
    2.6.1 :015 > answers = {}
     => {} 
    2.6.1 :016 > ans = answers.fetch(:x) {DEFAULT_BLOCK}
     => #<Proc:0x00005586f6ef9e58@(irb):14 (lambda)> 
    2.6.1 :017 > ans.call
     => "block executed" 

    Actually we can pass default value for key so that if key not found in the hash it use this default value like,
    my_hash = {}
     => {} 
    2.6.1 :019 > my_hash[:key1] = 'val1'
     => "val1" 
    2.6.1 :020 > p my_hash
    {:key1=>"val1"}
     => {:key1=>"val1"} 
    2.6.1 :022 > my_hash.fetch(:key1)
     => "val1" 
    2.6.1 :023 > my_hash.fetch(:key2)
    KeyError (key not found: :key2)
    Did you mean?  :key1
    2.6.1 :024 > my_hash.fetch(:key2){'val2'}
     => "val2"

I used it in irb of ruby version 2.6.1 – 2017kamb Feb 06 '19 at 10:42 — 2017kamb, Feb 06 '19 at 10:42

Using a lambda as a default in Hash#fetch ruby

3 Answers3