add let to frontend syntax

lib/Quox/Parser/FromParser.idr

@@ -264,6 +264,9 @@ mutual
         <*> fromPTermDScope ds ns [< j1] val1
         <*> pure loc
 
+    Let (qty, x, rhs) body loc =>
+      ?fromPTerm_let
+
   private
   fromPTermEnumArms : Loc -> Context' PatVar d -> Context' PatVar n ->
                       List (PTagVal, PTerm) ->

lib/Quox/Parser/Lexer.idr

@@ -261,6 +261,9 @@ reserved =
    Word "caseω" `Or` Word "case#",
    Word1 "return",
    Word1 "of",
+   Word1 "let", Word1 "in",
+   Word1 "let0", Word1 "let1",
+   Word "letω" `Or` Word "let#",
    Word1 "fst", Word1 "snd",
    Word1 "_",
    Word1 "Eq",

lib/Quox/Parser/Parser.idr

@@ -585,13 +585,28 @@ where
     foldr (\(q, x, s), t => Pi q x s t loc) cod $ toDoms (toQty q) doms
 
 
+letIntro : FileName -> Grammar True PQty
+letIntro fname =
+      withLoc fname (PQ Zero <$ res "let0")
+  <|> withLoc fname (PQ One  <$ res "let1")
+  <|> withLoc fname (PQ Any  <$ res "letω")
+  <|> do resC "let"
+         qty fname <* needRes "." <|> defLoc fname (PQ One)
 
 export
+letTerm : FileName -> Grammar True PTerm
+letTerm fname = withLoc fname $ do
+  qty  <- letIntro fname
+  x    <- patVar fname <* mustWork (resC "=")
+  rhs  <- assert_total term fname <* mustWork (resC "in")
+  body <- assert_total term fname
+  pure $ Let (qty, x, rhs) body
 
 -- term : FileName -> Grammar True PTerm
 term fname = lamTerm fname
          <|> piTerm fname
          <|> sigmaTerm fname
+         <|> letTerm fname
 
 
 export

lib/Quox/Parser/Syntax.idr

@@ -100,6 +100,8 @@ namespace PTerm
       | Coe (PatVar, PTerm) PDim PDim PTerm Loc
       | Comp (PatVar, PTerm) PDim PDim PTerm PDim
              (PatVar, PTerm) (PatVar, PTerm) Loc
+
+      | Let (PQty, PatVar, PTerm) PTerm Loc
     %name PTerm s, t
 
     public export
@@ -144,6 +146,7 @@ Located PTerm where
   (Ann     _ _           loc).loc = loc
   (Coe     _ _ _ _       loc).loc = loc
   (Comp    _ _ _ _ _ _ _ loc).loc = loc
+  (Let     _ _           loc).loc = loc
 
 export
 Located PCaseBody where