Minimal support for context.

Parse `context` keyword.

New Context constructor in Expr [API change].

Evaluate this by just evaluating the expression, for now.

Note that we don't support the features (like location or
numbering) that context is used to affect anyway, so this change
probably won't be enough for meaningful support. But it might
prevent some documents from issuing errors.

See #53.

src/Typst/Evaluate.hs

@@ -816,6 +816,8 @@ evalExpr expr =
         _ -> fail $ "For expression requires an Array or Dictionary"
       updateState $ \st -> st {evalFlowDirective = FlowNormal}
       go items VNone
+    Context e -> do
+      evalExpr e -- TODO for now we just ignore "context"
     Return mbe -> do
       -- these flow directives are examined in CodeBlock
       updateState (\st -> st {evalFlowDirective = FlowReturn (isJust mbe)})

src/Typst/Parse.hs

@@ -1067,6 +1067,7 @@ pKeywordExpr =
     <|> pBreakExpr
     <|> pContinueExpr
     <|> pReturnExpr
+    <|> pContextExpr
 
 -- args ::= ('(' (arg (',' arg)* ','?)? ')' content-block*) | content-block+
 pArgs :: P [Arg]
@@ -1235,5 +1236,8 @@ pReturnExpr = do
     then pure $ Return Nothing
     else Return <$> (option Nothing (Just <$> pExpr))
 
+pContextExpr :: P Expr
+pContextExpr = Context <$> (pKeyword "context" *> pExpr)
+
 pIncludeExpr :: P Expr
 pIncludeExpr = Include <$> (pKeyword "include" *> pExpr)

src/Typst/Syntax.hs

@@ -136,6 +136,7 @@ data Expr
   | Ident Identifier
   | FuncCall Expr [Arg]
   | FuncExpr [Param] Expr
+  | Context Expr
   | FieldAccess Expr Expr
   | Group Expr
   | Array [Spreadable Expr]