i should've made a lisp man this sucks

src/Rlp/Lex.x

@@ -10,6 +10,7 @@ module Rlp.Lex
     , lexStream
     , lexDebug
     , lexCont
+    , popLexState
     )
     where
 import Codec.Binary.UTF8.String (encodeChar)
@@ -73,6 +74,17 @@ $white_no_nl+       ;
 -- for the definition of `doBol`
 <0> \n                  { beginPush bol }
 
+<layout>
+{
+    
+}
+
+-- layout keywords
+<0>
+{
+    "let"               { constToken TokenLet `thenBeginPush` layout_let }
+}
+
 -- scan various identifiers and reserved words. order is important here!
 <0>
 {
@@ -110,6 +122,14 @@ $white_no_nl+       ;
     ()                  { doBol }
 }
 
+<layout_let>
+{
+    \n                  { beginPush bol }
+    "{"                 { explicitLBrace }
+    "in"                { constToken TokenIn `thenDo` (popLexState *> popLayout) }
+    ()                  { doLayout }
+}
+
 <layout_top>
 {
     \n                  ;
@@ -144,6 +164,12 @@ thenBegin act c inp l = do
     psLexState . _head .= c
     pure a
 
+thenBeginPush :: LexerAction a -> Int -> LexerAction a
+thenBeginPush act c inp l = do
+    a <- act inp l
+    pushLexState c
+    pure a
+
 andBegin :: LexerAction a -> Int -> LexerAction a
 andBegin act c inp l = do
     psLexState . _head .= c
@@ -342,6 +368,7 @@ explicitRBrace inp l = do
 doLayout :: LexerAction (Located RlpToken)
 doLayout _ _ = do
     i <- indentLevel
+    traceM $ "doLayout: i: " <> show i
     pushLayout (Implicit i)
     popLexState
     insertLBrace

src/Rlp/Parse.y

@@ -2,6 +2,7 @@
 {-# LANGUAGE LambdaCase, ViewPatterns #-}
 module Rlp.Parse
     ( parseRlpProg
+    , parseRlpExpr
     )
     where
 import Compiler.RlpcError
@@ -23,6 +24,7 @@ import Data.Void
 }
 
 %name parseRlpProg StandaloneProgram
+%name parseRlpExpr StandaloneExpr
 
 %monad { P }
 %lexer { lexCont } { Located _ TokenEOF }
@@ -51,8 +53,12 @@ import Data.Void
     infixl          { Located _ TokenInfixL }
     infixr          { Located _ TokenInfixR }
     infix           { Located _ TokenInfix }
+    let             { Located _ TokenLet }
+    in              { Located _ TokenIn }
 
+%nonassoc '='
 %right '->'
+%right in
 
 %%
 
@@ -60,6 +66,9 @@ StandaloneProgram   :: { RlpProgram RlpcPs }
 StandaloneProgram   : '{' Decls  '}'        {% mkProgram $2 }
                     | VL  DeclsV VR         {% mkProgram $2 }
 
+StandaloneExpr      :: { RlpExpr RlpcPs }
+                    : VL Expr VR            { extract $2 }
+
 VL  :: { () }
 VL  : vlbrace       { () }
 
@@ -143,6 +152,20 @@ Expr        :: { RlpExpr' RlpcPs }
             : Expr1 InfixOp Expr        { $2 =>> \o ->
                                           OAppE (extract o) $1 $3 }
             | Expr1                     { $1 }
+            | LetExpr                   { $1 }
+
+LetExpr     :: { RlpExpr' RlpcPs }
+            : let layout1(Binding) in Expr { $1 \$> LetE $2 $4 }
+
+layout1(p)  : '{' layout_list1(';',p) '}'   { $2 }
+            | VL  layout_list1(VS,p) VR     { $2 }
+
+layout_list1(sep,p) : p sep                      { [$1] }
+                    | p                          { [$1] }
+                    | layout_list1(sep,p) sep p  { $1 `snoc` $3 }
+
+Binding     :: { Binding' RlpcPs }
+            : Pat1 '=' Expr              { PatB <<~ $1 <~> $3 }
 
 Expr1       :: { RlpExpr' RlpcPs }
             : '(' Expr ')'              { $1 .> $2 <. $3 }

src/Rlp/Syntax.hs

@@ -14,6 +14,7 @@ module Rlp.Syntax
     , Lit(..), Lit'
     , RlpType(..), RlpType'
     , ConAlt(..)
+    , Binding(..), Binding'
 
     -- * Trees That Grow extensions
     , UnXRec(..), MapXRec(..), XRec, IdP
@@ -28,7 +29,7 @@ import Data.Functor.Classes
 import Data.Kind                    (Type)
 import Lens.Micro
 import Lens.Micro.TH
-import Core.Syntax                  hiding (Lit, Type, Binding)
+import Core.Syntax                  hiding (Lit, Type, Binding, Binding')
 import Core                         (HasRHS(..), HasLHS(..))
 ----------------------------------------------------------------------------------
 
@@ -42,6 +43,7 @@ type PhaseShow p =
     ( Show (XRec p Pat), Show (XRec p RlpExpr)
     , Show (XRec p Lit), Show (IdP p)
     , Show (XRec p RlpType)
+    , Show (XRec p Binding)
     )
 
 newtype RlpProgram p = RlpProgram [Decl' p]
@@ -77,7 +79,7 @@ data ConAlt p = ConAlt (IdP p) [RlpType' p]
 
 deriving instance (Show (IdP p), Show (XRec p RlpType)) => Show (ConAlt p)
 
-data RlpExpr p = LetE  [Binding p] (RlpExpr' p)
+data RlpExpr p = LetE  [Binding' p] (RlpExpr' p)
                | VarE  (IdP p)
                | LamE  [Pat p] (RlpExpr' p)
                | CaseE (RlpExpr' p) [(Alt p, Where p)]
@@ -111,6 +113,8 @@ deriving instance (PhaseShow p) => Show (Alt p)
 data Binding p = PatB (Pat' p) (RlpExpr' p)
                | FunB (IdP p) [Pat' p] (RlpExpr' p)
 
+type Binding' p = XRec p Binding
+
 deriving instance (Show (XRec p Pat), Show (XRec p RlpExpr), Show (IdP p)
                   ) => Show (Binding p)