partial evaluation for (+)

src/compiler/GF/Compile/Compute/Concrete.hs

@@ -126,6 +126,11 @@ eval env (C t1 t2)      []  = do v1 <- eval env t1 []
                                    (VC vs1,v2    ) -> return (VC (vs1++[v2]))
                                    (v1,    VC vs2) -> return (VC ([v1]++vs2))
                                    (v1,    v2    ) -> return (VC [v1,v2])
+eval env t@(Glue t1 t2) []  = do v1 <- eval env t1 []
+                                 v2 <- eval env t2 []
+                                 case liftM2 (++) (value2string v1) (value2string v2) of
+                                   Just s  -> return (string2value s)
+                                   Nothing -> evalError ("Cannot reduce term" <+> pp t)
 eval env (FV ts)        vs  = msum [eval env t vs | t <- ts]
 eval env (Error msg)    vs  = fail msg
 eval env t              vs  = evalError ("Cannot reduce term" <+> pp t)

testsuite/compiler/compute/predef.gfs

@@ -31,3 +31,5 @@ cc "x"++SOFT_BIND++"y"
 cc "x"++SOFT_SPACE++"y"
 cc "x"++CAPIT++"y"
 cc "x"++ALL_CAPIT++"y"
+cc "a"+"b"
+cc eqInt (length ("a"+"b")) 2

testsuite/compiler/compute/predef.gfs.gold

@@ -32,3 +32,5 @@ CallStack (from HasCallStack):
 "x" ++ Predef.SOFT_SPACE ++ "y"
 "x" ++ Predef.CAPIT ++ "y"
 "x" ++ Predef.ALL_CAPIT ++ "y"
+"ab"
+Predef.PTrue