[RFC] add isDefault; more general than isNil, isEmpty etc

changelog.md

@@ -81,6 +81,7 @@
 
 
 ```
+- Added `system.isDefault(a)` to tell whether `a` is equal to its default value
 
 
 ## Library changes

lib/system.nim

@@ -479,6 +479,24 @@ let nimvm* {.magic: "Nimvm", compileTime.}: bool = false
 include "system/arithmetics"
 include "system/comparisons"
 
+const
+  NimMajor* {.intdefine.}: int = 1
+    ## is the major number of Nim's version.
+
+  NimMinor* {.intdefine.}: int = 1
+    ## is the minor number of Nim's version.
+
+  NimPatch* {.intdefine.}: int = 1
+    ## is the patch number of Nim's version.
+
+template since2(version, body: untyped) {.dirty.} =
+  ## poor-man's version of inclrtl.since without `>=` defined
+  const
+    v0 = version[0]
+    v1 = version[1]
+  when NimMajor > v0 or NimMajor == v0 and NimMinor >= v1:
+    body
+
 const
   appType* {.magic: "AppType"}: string = ""
     ## A string that describes the application type. Possible values:
@@ -909,6 +927,43 @@ else:
   else:
     proc reset*[T](obj: var T) {.magic: "Reset", noSideEffect.}
 
+since2 (1, 1):
+  template isDefault*[T](a: T): bool =
+    ## returns whether `a` is equal to its default value
+    runnableExamples:
+      doAssert "".isDefault
+      doAssert not "a".isDefault
+      doAssert not @[0].isDefault
+
+      ## semantic differences with something like isEmpty:
+      doAssert [0].isDefault # whereas [0].len != 0
+      doAssert not "".cstring.isDefault # whereas "".cstring.len == 0
+
+      type Kind = enum kUnknown, kRed, kGreen
+      doAssert kUnknown.isDefault
+      doAssert (-0.0).isDefault
+
+      type Foo1 = ref object
+      type Foo2 = object
+      doAssert not Foo1().isDefault
+      doAssert Foo1().notDefault
+      doAssert Foo2().isDefault
+
+    # pending https://github.com/nim-lang/Nim/issues/13527, use `default(T)`
+    a == default(type(a))
+
+  template isDefault*(a: string): bool =
+    ## overloaded for efficiency
+    a.len == 0
+
+  template isDefault*[T](a: seq[T]): bool =
+    ## overloaded for efficiency
+    a.len == 0
+
+  template notDefault*(a): untyped =
+    ## negation of `isDefault`
+    not isDefault(a)
+
 proc setLen*[T](s: var seq[T], newlen: Natural) {.
   magic: "SetLengthSeq", noSideEffect.}
   ## Sets the length of seq `s` to `newlen`. ``T`` may be any sequence type.
@@ -2049,15 +2104,6 @@ import system/dollars
 export dollars
 
 const
-  NimMajor* {.intdefine.}: int = 1
-    ## is the major number of Nim's version.
-
-  NimMinor* {.intdefine.}: int = 1
-    ## is the minor number of Nim's version.
-
-  NimPatch* {.intdefine.}: int = 1
-    ## is the patch number of Nim's version.
-
   NimVersion*: string = $NimMajor & "." & $NimMinor & "." & $NimPatch
     ## is the version of Nim as a string.
 

tests/errmsgs/twrong_at_operator.nim

@@ -4,14 +4,14 @@ line: 22
 nimout: '''
 twrong_at_operator.nim(22, 30) Error: type mismatch: got <array[0..0, type int]>
 but expected one of:
-proc `@`[IDX, T](a: sink array[IDX, T]): seq[T]
-  first type mismatch at position: 1
-  required type for a: sink array[IDX, T]
-  but expression '[int]' is of type: array[0..0, type int]
 proc `@`[T](a: openArray[T]): seq[T]
   first type mismatch at position: 1
   required type for a: openArray[T]
   but expression '[int]' is of type: array[0..0, type int]
+proc `@`[IDX, T](a: sink array[IDX, T]): seq[T]
+  first type mismatch at position: 1
+  required type for a: sink array[IDX, T]
+  but expression '[int]' is of type: array[0..0, type int]
 
 expression: @[int]
 '''