How to read query string parameters

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livery_ext:query/2 reads a value from the request URL's query string. You need it whenever a handler depends on parameters like ?q=... or ?page=....

Read a parameter

search(Req) ->
    Q = livery_ext:query(<<"q">>, Req),
    Page = livery_ext:query(<<"page">>, Req),
    do_search(Q, Page).

livery_ext:query/2 returns the first value for the key, or undefined if it is missing.

Apply a default value

Wrap with a fallback:

Page = case livery_ext:query(<<"page">>, Req) of
    undefined -> <<"1">>;
    V         -> V
end.

Convert to an integer

livery_ext:query/2 always returns a binary. Convert at the call site:

PageInt = try binary_to_integer(Page) catch _:_ -> 1 end.

URL-decoded values

Values are URL-decoded:

%% /search?q=hello%20world&unit=100%25
<<"hello world">> = livery_ext:query(<<"q">>, Req),
<<"100%">>        = livery_ext:query(<<"unit">>, Req).

Notes

  • livery_ext:query/2 returns only the first value for a repeated key. To read all values, call livery_req:query/1 to get the raw query string and parse it yourself, or open an issue if you need this in livery_ext.

See also

  • Reference: livery_ext
  • Reference: livery_req