improve rdstdin: fix genode bug, remove duplication, improve docs

lib/impure/rdstdin.nim

@@ -22,38 +22,24 @@ runnableExamples("-r:off"):
     if line.len > 0: echo line
   echo "exiting"
 
-when defined(windows):
-  proc readLineFromStdin*(prompt: string): string {.
-                          tags: [ReadIOEffect, WriteIOEffect].} =
-    ## Reads a line from stdin.
-    stdout.write(prompt)
-    result = readLine(stdin)
-
-  proc readLineFromStdin*(prompt: string, line: var string): bool {.
-                          tags: [ReadIOEffect, WriteIOEffect].} =
-    ## Reads a `line` from stdin. `line` must not be
-    ## `nil`! May throw an IO exception.
-    ## A line of text may be delimited by `CR`, `LF` or
-    ## `CRLF`. The newline character(s) are not part of the returned string.
-    ## Returns `false` if the end of the file has been reached, `true`
-    ## otherwise. If `false` is returned `line` contains no new data.
-    stdout.write(prompt)
-    result = readLine(stdin, line)
+const useLinenoise = not (defined(windows) or defined(genode))
 
-elif defined(genode):
-  proc readLineFromStdin*(prompt: string): string {.
-                          tags: [ReadIOEffect, WriteIOEffect].} =
-    stdin.readLine()
-
-  proc readLineFromStdin*(prompt: string, line: var string): bool {.
-                          tags: [ReadIOEffect, WriteIOEffect].} =
-    stdin.readLine(line)
-
-else:
+when useLinenoise:
   import linenoise
 
-  proc readLineFromStdin*(prompt: string, line: var string): bool {.
-                          tags: [ReadIOEffect, WriteIOEffect].} =
+proc readLineFromStdin*(prompt: string, line: var string): bool {.
+                        tags: [ReadIOEffect, WriteIOEffect].} =
+  ## Reads a `line` from stdin. May raise `IOError`.
+  ##
+  ## A line of text may be delimited by ``\r``, ``\n`` or ``\r\n``, which are not
+  ## part of the returned string.
+  ##
+  ## On platforms that support `linenoise`, it will be used.
+  ##
+  ## Returns `false` if the end of the file has been reached (or `^C` was entered),
+  ## in which case `line.len == 0`. Entering an empty string would still return
+  ## true.
+  when useLinenoise:
     var buffer = linenoise.readLine(prompt)
     if isNil(buffer):
       line.setLen(0)
@@ -63,7 +49,11 @@ else:
       historyAdd(buffer)
     linenoise.free(buffer)
     result = true
+  else:
+    stdout.write(prompt)
+    result = readLine(stdin, line)
 
-  proc readLineFromStdin*(prompt: string): string {.inline.} =
-    if not readLineFromStdin(prompt, result):
-      raise newException(IOError, "Linenoise returned nil")
+proc readLineFromStdin*(prompt: string): string {.inline.} =
+  ## Outplace overload.
+  if not readLineFromStdin(prompt, result):
+    raise newException(IOError, "no data returned by `readLineFromStdin`")