Merge pull request #2437 from Araq/underscore-tuple-unpack

Implements #2154.

compiler/lexer.nim

@@ -61,7 +61,7 @@ type
     tkComma, tkSemiColon,
     tkColon, tkColonColon, tkEquals, tkDot, tkDotDot,
     tkOpr, tkComment, tkAccent,
-    tkSpaces, tkInfixOpr, tkPrefixOpr, tkPostfixOpr,
+    tkSpaces, tkInfixOpr, tkPrefixOpr, tkPostfixOpr
 
   TTokTypes* = set[TTokType]
 
@@ -863,6 +863,10 @@ proc rawGetTok*(L: var TLexer, tok: var TToken) =
     of '`':
       tok.tokType = tkAccent
       inc(L.bufpos)
+    of '_':
+      tok.tokType = tkSymbol
+      tok.ident = getIdent("_")
+      inc(L.bufpos)
     of '\"':
       # check for extended raw string literal:
       var rawMode = L.bufpos > 0 and L.buf[L.bufpos-1] in SymChars

compiler/semstmts.nim

@@ -369,6 +369,10 @@ proc addToVarSection(c: PContext; result: var PNode; orig, identDefs: PNode) =
   else:
     result.add identDefs
 
+proc isDiscardUnderscore(n: PNode): bool =
+  if n.kind != nkIdent: return false
+  return n.ident.s == "_"
+
 proc semVarOrLet(c: PContext, n: PNode, symkind: TSymKind): PNode =
   var b: PNode
   result = copyNode(n)
@@ -432,7 +436,10 @@ proc semVarOrLet(c: PContext, n: PNode, symkind: TSymKind): PNode =
 
     for j in countup(0, length-3):
       var v = semIdentDef(c, a.sons[j], symkind)
-      if sfGenSym notin v.flags: addInterfaceDecl(c, v)
+      if sfGenSym notin v.flags and
+         not isDiscardUnderscore(a.sons[j]): addInterfaceDecl(c, v)
+      if isDiscardUnderscore(a.sons[j]):
+        v.flags.incl(sfGenSym)
       when oKeepVariableNames:
         if c.inUnrolledContext > 0: v.flags.incl(sfShadowed)
         else:

tests/parser/ttupleunpack.nim

@@ -0,0 +1,29 @@
+discard """
+  file: "ttupleunpack.nim"
+  output: ""
+  exitcode: 0
+"""
+
+proc main() =
+
+  proc foo(): tuple[x, y, z: int] =
+    return (4, 2, 3)
+
+  var (x, _, y) = foo()
+  doAssert x == 4
+  doAssert y == 3
+
+  var (a, _, _) = foo()
+  doAssert a == 4
+
+  var (a, _, _xx) = foo()
+  doAssert a == 4
+
+  iterator bar(): tuple[x, y, z: int] =
+    yield (1,2,3)
+
+  for x, y, _ in bar():
+    doAssert x == 1
+    doAssert y == 2
+
+main()

web/news.txt

@@ -129,6 +129,8 @@ News
     of the assignment operator has arrived!
   - ``system.len`` for strings and sequences now returns 0 for nil.
 
+  - A single underscore can now be used to discard values when unpacking tuples.
+
 
   Library additions
   -----------------