Guard clauses in Elixir allow us to extend the standard parameter pattern match to evaluating against a predicate.<\/p>\n
For example<\/p>\n
\r\ndefmodule Guard do\r\n def check(value) when value == nil do\r\n IO.puts "Value is nil"\r\n end\r\n\r\n def check(value) when is_integer(value) or is_float(value) do\r\n IO.puts "#{value} is number"\r\n end\r\n\r\n def check(value) when is_atom(value) do\r\n IO.puts "#{value} is atom"\r\n end\r\n\r\nend\r\n<\/pre>\nWe can see the use of the when<\/strong> keyword, also for multiple guards we use the or<\/em> keyword. The right side of the when<\/em> clauses is a predicate, hence should return true or false.<\/p>\nIn these examples if we enter Guard.check(nil)<\/em> the result is “Value is nil” if you check 123 or 123.4 (for example) the second guarded function is called etc. <\/p>\nNow one must remember that Elixir will evaluate functions from the top to the bottom, hence if the is_atom<\/em> check is moved to the top of the functions, a nil<\/em> will match as an atom and hence never reach the “Value is nil” returning function.<\/p>\nAs we’ve seen, we supply a predicate to the function which means we can check the value for things like it’s type. We can also handle ranges, for example maybe we have one function that handle division but guards against a denominator of 0, then we might create two functions like this<\/p>\n
\r\ndef div(a, b) when b == 0 do\r\n if b == 0, do: raise("Divide by Zero")\r\nend\r\n\r\ndef div(a, b) do\r\n a \/ b\r\nend\r\n<\/pre>\nI’m not saying this is better than writing something like the code below, but it’s another way of doing things<\/p>\n
\r\ndef div(a, b) do\r\n if b == 0, do: raise("Divide by Zero")\r\n\r\n a \/ b\r\nend\r\n<\/pre>\nAs we saw, we use or<\/em> not ||<\/em>. <\/p>\n\n- The boolean operators allowed are or<\/em>, and<\/em>, not<\/em> and !<\/em>.<\/li>\n
- Comparison operators include ==<\/em>, !=<\/em>, ===<\/em>, ><\/em>, <<\/em>, >=<\/em> and <=<\/em>.<\/li>\n
- Arithmetic operators +<\/em>, –<\/em>, *<\/em>, \/<\/em> can be used.<\/li>\n
- Join operators <><\/em> and ++<\/em> can be used.<\/li>\n
- The in<\/em> operator can be used.<\/li>\n
- Type check functions such as is_integer<\/em> etc.<\/li>\n<\/ul>\n