Define ordering and equality on Char

examples/eval-tests.dx

@@ -771,6 +771,21 @@ s1 = "hello world"
 :p s1
 > ['h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']
 
+:p codepoint 'a'
+> 97
+
+:p 'a' == 'a'
+> True
+
+:p 'a' == 'A'
+> False
+
+:p 'a' < 'b'
+> True
+
+:p 'a' > 'b'
+> False
+
 :p
   x = 2 + 2
   y = 2 + 4

prelude.dx

@@ -545,7 +545,6 @@ def argmin (_:Ord o) ?=> (xs:n=>o) : n =
   zipped = for i. (i, xs.i)
   fst $ reduce zeroth compare zipped
 
-
 'Automatic differentiation
 
 -- TODO: add vector space constraints
@@ -793,6 +792,8 @@ splitV : Iso a ({|} | a) =
 def sliceFields (iso: Iso ({|} | a) (b | c)) (tab: a=>v) : b=>v =
   reindex (buildWith $ splitV &>> iso) tab
 
+'## Strings and Characters
+
 Char : Type = %Char
 def MkChar (c:Int8) : Char = %MkChar c
 
@@ -803,6 +804,14 @@ CharPtr : Type = %CharPtr
 interface Show a:Type where
   show : a -> String
 
+-- TODO. This is ASCII code point. It really should be Int32 for Unicode codepoint 
+def codepoint (c:Char) : Int8 = %codePoint c
+
+@instance charEq  : Eq Char  = MkEq \x y. codepoint x == codepoint y
+@instance charOrd : Ord Char = (MkOrd charEq (\x y. codepoint x > codepoint y)
+                                             (\x y. codepoint x < codepoint y))
+
+
 def showFloat' (x:Float) : String =
   (n, ptr) = %ffi showFloat (Int & CharPtr) x
   AsList n $ for i:(Fin n).
@@ -811,7 +820,6 @@ def showFloat' (x:Float) : String =
 instance showFloat : Show Float where
   show = showFloat'
 
-
 '## Floating point helper functions
 
 def sign (x:Float) : Float =

src/lib/Autodiff.hs

@@ -150,6 +150,7 @@ linearizeOp op = case op of
   UnsafeFromOrdinal _ _  -> emitDiscrete
   ToOrdinal _            -> emitDiscrete
   IdxSetSize _           -> emitDiscrete
+  CodePoint _           -> emitDiscrete
   ThrowError _           -> emitWithZero
   CastOp t v             -> do
     if tangentType vt == vt && tangentType t == t
@@ -627,8 +628,9 @@ transposeOp op ct = case op of
   SliceOffset  _ _      -> notLinear
   SliceCurry   _ _      -> notLinear
   UnsafeFromOrdinal _ _ -> notLinear
-  ToOrdinal   _         -> notLinear
+  ToOrdinal    _        -> notLinear
   IdxSetSize   _        -> notLinear
+  CodePoint    _        -> notLinear
   ThrowError   _        -> notLinear
   FFICall      _ _ _    -> notLinear
   where

src/lib/Imp.hs

@@ -244,6 +244,7 @@ toImpOp (maybeDest, op) = case op of
     tileOffset' <- iaddI (fromScalarAtom tileOffset) extraOffset
     returnVal $ toScalarAtom tileOffset'
   ThrowError _ -> throwError ()
+  CodePoint ~(Con (CharCon x)) -> returnVal x
   CastOp destTy x -> case (getType x, destTy) of
     (BaseTy _, BaseTy bt) -> returnVal =<< toScalarAtom <$> cast (fromScalarAtom x) bt
     _ -> error $ "Invalid cast: " ++ pprint (getType x) ++ " -> " ++ pprint destTy

src/lib/Syntax.hs

@@ -332,6 +332,7 @@ data PrimOp e =
       | ToOrdinal e
       | IdxSetSize e
       | ThrowError e
+      | CodePoint e
       | CastOp e e                   -- Type, then value. See Type.hs for valid coercions.
       -- Extensible record and variant operations:
       -- Add fields to a record (on the left). Left arg contains values to add.
@@ -1473,7 +1474,8 @@ builtinNames = M.fromList
   , ("vfadd", vbinOp FAdd), ("vfsub", vbinOp FSub), ("vfmul", vbinOp FMul)
   , ("idxSetSize"  , OpExpr $ IdxSetSize ())
   , ("unsafeFromOrdinal", OpExpr $ UnsafeFromOrdinal () ())
-  , ("toOrdinal"        , OpExpr $ ToOrdinal ())
+  , ("toOrdinal"        , OpExpr $ ToOrdinal ())  
+  , ("codePoint" , OpExpr $ CodePoint ())
   , ("throwError" , OpExpr $ ThrowError ())
   , ("ask"        , OpExpr $ PrimEffect () $ MAsk)
   , ("tell"       , OpExpr $ PrimEffect () $ MTell ())

src/lib/Type.hs

@@ -728,6 +728,10 @@ typeCheckOp op = case op of
     i |: TC (IntRange (IdxRepVal 0) (IdxRepVal $ fromIntegral vectorWidth))
     return $ BaseTy $ Scalar sb
   ThrowError ty -> ty|:TyKind $> ty
+  -- TODO: this should really be a 32 bit integer for unicode code point: but for now is 8 bit ASCII code point
+  CodePoint c -> do
+    c |: CharTy
+    return $ BaseTy $ Scalar Int8Type
   CastOp t@(Var _) _ -> t |: TyKind $> t
   CastOp destTy e -> do
     sourceTy <- typeCheck e