Block expressions (#6695)

* enabled parsing of block expressions
* added test case; closes #3827
* adjusted grammar doc strings
* updated grammar in docs

compiler/parser.nim

@@ -72,6 +72,7 @@ proc parseStmtPragma(p: var TParser): PNode
 proc parsePragma(p: var TParser): PNode
 proc postExprBlocks(p: var TParser, x: PNode): PNode
 proc parseExprStmt(p: var TParser): PNode
+proc parseBlock(p: var TParser): PNode
 # implementation
 
 proc getTok(p: var TParser) =
@@ -1078,12 +1079,14 @@ proc parseTypeDescKAux(p: var TParser, kind: TNodeKind,
     parseSymbolList(p, list)
 
 proc parseExpr(p: var TParser): PNode =
-  #| expr = (ifExpr
+  #| expr = (blockExpr
+  #|       | ifExpr
   #|       | whenExpr
   #|       | caseExpr
   #|       | tryExpr)
   #|       / simpleExpr
   case p.tok.tokType:
+  of tkBlock: result = parseBlock(p)
   of tkIf: result = parseIfExpr(p, nkIfExpr)
   of tkWhen: result = parseIfExpr(p, nkWhenExpr)
   of tkCase: result = parseCase(p)
@@ -1517,6 +1520,7 @@ proc parseFor(p: var TParser): PNode =
 
 proc parseBlock(p: var TParser): PNode =
   #| blockStmt = 'block' symbol? colcom stmt
+  #| blockExpr = 'block' symbol? colcom stmt
   result = newNodeP(nkBlockStmt, p)
   getTokNoInd(p)
   if p.tok.tokType == tkColon: addSon(result, ast.emptyNode)

doc/grammar.txt

@@ -83,7 +83,8 @@ paramListColon = paramList? (':' optInd typeDesc)?
 doBlock = 'do' paramListArrow pragmas? colcom stmt
 procExpr = 'proc' paramListColon pragmas? ('=' COMMENT? stmt)?
 distinct = 'distinct' optInd typeDesc
-expr = (ifExpr
+expr = (blockExpr
+      | ifExpr
       | whenExpr
       | caseExpr
       | tryExpr)
@@ -141,6 +142,7 @@ tryExpr = 'try' colcom stmt &(optInd 'except'|'finally')
 exceptBlock = 'except' colcom stmt
 forStmt = 'for' (identWithPragma ^+ comma) 'in' expr colcom stmt
 blockStmt = 'block' symbol? colcom stmt
+blockExpr = 'block' symbol? colcom stmt
 staticStmt = 'static' colcom stmt
 deferStmt = 'defer' colcom stmt
 asmStmt = 'asm' pragma? (STR_LIT | RSTR_LIT | TRIPLE_STR_LIT)

tests/parser/tletcolon.nim

@@ -4,7 +4,8 @@ discard """
 44 3
 more body code
 yes
-yes'''
+yes
+block expression works'''
 """
 
 template x(body): untyped =
@@ -50,3 +51,11 @@ let format =
     else:
         echo "Texture Format Unknown, assuming RGB"  #This echo causes an error
         TextureInternalFormat.RGB
+
+# Block as expressions #3827
+block:
+  let x = block:
+    var y = 2
+    echo "block expression works"
+    y*y
+  doAssert x == 4