Report the municipality a missing CEP would have belonged toHEADv0.2.0main

A CEP that fails a lookup is rarely a random number: it is usually someone
who knows the city and guessed, and the classic guess is the -000 that 957
municipalities never received. The API now answers those 404s with a faixa
naming the municipality that owns the band the number fell into, which is
the difference between telling a user "invalid CEP" and "no such CEP in
Areal/RJ".

It is context for an error and never an address, so it rides on the
cepis_not_found condition instead of turning the failure into a success,
and cep_ini/cep_fim are the endpoints that exist rather than the range the
Correios publish - the interval between them is sparse, which is what ceps
counts. Parsing is deliberately unable to signal: a faixa that is null, an
array, a string or half a body leaves a clean not-found behind, because the
caller asked for the CEP, not for the decoration on its absence.

1 files changed, 12 insertions, 2 deletions

diff --git a/R/http.R b/R/http.R
index 6ef4511..08e8ddc 100644
--- a/R/http.R
+++ b/R/http.R
@@ -173,12 +173,22 @@ json_object <- function(body) {
   data
 }
 
-api_message <- function(body, fallback) {
+# The counterpart of json_object() for a body that is already an error: it
+# never signals, because a broken error body must not replace the error the
+# caller is being told about. Missing, malformed, empty: all of them NULL.
+json_field <- function(body, name) {
   data <- tryCatch(
     jsonlite::parse_json(decode_body(body), simplifyVector = FALSE),
     error = function(e) NULL
   )
-  message <- if (is.list(data)) data[["mensagem"]] else NULL
+  if (!is.list(data) || is.null(names(data))) {
+    return(NULL)
+  }
+  data[[name]]
+}
+
+api_message <- function(body, fallback) {
+  message <- json_field(body, "mensagem")
   if (is.character(message) && length(message) == 1L && nzchar(message)) {
     message
   } else {