[WIP] fix #nim-lang/RFCs/issues/84 generic parse

compiler/conv.nim

@@ -0,0 +1,85 @@
+#[
+unfinished
+generic parser
+]#
+
+import parseutils
+from strutils import startsWith, continuesWith
+
+template enforce(a: untyped, msg: untyped) =
+  # todo: custom catchable exception
+  doAssert a, $msg
+
+proc skipWhitespace(src: string, start: var int) =
+  while start < src.len and src[start] == ' ': # CHECKME for ws
+    start.inc
+
+template skipWhitespace2() =
+  skipWhitespace(src, start)
+
+template continuesWith2(pattern: string): bool=
+  src.continuesWith(pattern, start)
+
+
+template continuesWithAndAdvance(pattern: string): bool=
+  let old = start
+  skipWhitespace2()
+  if src.continuesWith(pattern, start):
+    start.inc pattern.len
+    true
+  else:
+    start = old
+    false
+
+proc eat(src: string, start: var int, pattern: string) =
+  enforce continuesWithAndAdvance(pattern), (src, start, pattern)
+
+proc parseFrom*[T](dst: var T, src: string, start: var int) =
+  template process(parseCustom) =
+    let ret = parseCustom(src, dst, start)
+    enforce ret > 0, (start, src)
+    start += ret
+
+  skipWhitespace2()
+  when T is seq:
+    parseFrom(dst, src, start)
+  elif T is int: process(parseInt)
+  elif T is float: process(parseFloat)
+  elif T is bool:
+    if continuesWithAndAdvance("true"): dst = true
+    elif continuesWithAndAdvance("false"): dst = false
+    else:
+      # else: enforce false, ($T, start, src) # nim BUG cgen error
+      doAssert false, $($T, start)
+  else:
+    static: doAssert false, $T
+
+proc parseFrom*[T](dst: var seq[T], src: string, start: var int) =
+  eat(src, start, "@[")
+  while true:
+    if continuesWithAndAdvance("]"): return
+    var a: T
+    parseFrom(a, src, start)
+    dst.add a
+    if continuesWithAndAdvance(","):
+      continue
+    else:
+      break
+  eat(src, start, "]")
+
+proc parseFrom*[T](dst: var T, src: string) =
+  var start = 0
+  parseFrom(dst, src, start)
+  enforce start == src.len, (start, src.len)
+
+proc parse*[T](src: string): T =
+  parseFrom(result, src)
+
+when isMainModule:
+  proc runTest[T](a: T) =
+    doAssert a == parse[T]($a), $(a, $a, $T)
+  runTest 123
+  runTest 3.14
+  runTest @[1,2]
+  runTest @[@[1,2], @[3], @[]]
+  runTest @[true, false]