From fc0ede9499a1a768729bb8fe954b6618df05ee15 Mon Sep 17 00:00:00 2001
From: Joscha <joscha@plugh.de>
Date: Thu, 26 Nov 2020 14:16:23 +0100
Subject: [PATCH] Create basic lambda term type

---
 package.yaml        |  2 ++
 props.cabal         |  7 ++++++-
 src/Props/Lambda.hs | 23 +++++++++++++++++++++++
 3 files changed, 31 insertions(+), 1 deletion(-)
 create mode 100644 src/Props/Lambda.hs

diff --git a/package.yaml b/package.yaml
index d1f1575..5062cc2 100644
--- a/package.yaml
+++ b/package.yaml
@@ -13,6 +13,8 @@ extra-doc-files:
 
 dependencies:
 - base >= 4.7 && < 5
+- containers
+- text
 
 library:
   source-dirs: src
diff --git a/props.cabal b/props.cabal
index 8063c21..bc459b3 100644
--- a/props.cabal
+++ b/props.cabal
@@ -4,7 +4,7 @@ cabal-version: 1.18
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 057a536ac3d0fd40b3122084eee53201158527bbcd0dbac7cec33d8fd06cf208
+-- hash: 5e16e68a3016455a93f44cbee36b19144a1a414a8cb0bf4a062a1566b7119a38
 
 name:           props
 version:        0.1.0.0
@@ -23,12 +23,15 @@ extra-doc-files:
 library
   exposed-modules:
       Props
+      Props.Lambda
   other-modules:
       Paths_props
   hs-source-dirs:
       src
   build-depends:
       base >=4.7 && <5
+    , containers
+    , text
   default-language: Haskell2010
 
 executable props
@@ -40,5 +43,7 @@ executable props
   ghc-options: -threaded -rtsopts -with-rtsopts=-N
   build-depends:
       base >=4.7 && <5
+    , containers
     , props
+    , text
   default-language: Haskell2010
diff --git a/src/Props/Lambda.hs b/src/Props/Lambda.hs
new file mode 100644
index 0000000..1b71ce6
--- /dev/null
+++ b/src/Props/Lambda.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Props.Lambda
+  ( Term(..)
+  , displayTerm
+  ) where
+
+import qualified Data.Set  as Set
+import qualified Data.Text as T
+
+type Name = T.Text
+type PreferredName = Maybe Name
+
+-- | Lambda calculus term using De Bruijn indexing
+data Term a
+  = Var PreferredName Int
+  | Lambda PreferredName (Term a)
+  | App (Term a) (Term a)
+  | Native a
+  deriving (Show)
+
+displayTerm :: (Set.Set Name -> a -> T.Text) -> Term a -> T.Text
+displayTerm = undefined