Three improvements to Template Haskell (fixes #3467)

[ghc-hetmet.git] / compiler / typecheck / TcRnTypes.lhs

diff --git a/compiler/typecheck/TcRnTypes.lhs b/compiler/typecheck/TcRnTypes.lhs
index cbc443f..c011d20 100644 (file)
--- a/compiler/typecheck/TcRnTypes.lhs
+++ b/compiler/typecheck/TcRnTypes.lhs
@@ -22,7 +22,7 @@ module TcRnTypes(
 
        -- Template Haskell
        ThStage(..), topStage, topAnnStage, topSpliceStage,
-       ThLevel, impLevel, topLevel,
+       ThLevel, impLevel, outerLevel, thLevel,
 
        -- Arrows
        ArrowCtxt(NoArrowCtxt), newArrowScope, escapeArrowScope,
@@ -382,37 +382,55 @@ pass it inwards.
 -}
 
 ---------------------------
--- Template Haskell levels 
+-- Template Haskell stages and levels 
 ---------------------------
 
+data ThStage   -- See Note [Template Haskell state diagram] in TcSplice
+  = Splice     -- Top-level splicing
+               -- This code will be run *at compile time*;
+               --   the result replaces the splice
+               -- Binding level = 0
+ 
+  | Comp       -- Ordinary Haskell code
+               -- Binding level = 1
+
+  | Brack                      -- Inside brackets 
+      ThStage                  --   Binding level = level(stage) + 1
+      (TcRef [PendingSplice])  --   Accumulate pending splices here
+      (TcRef LIE)              --     and type constraints here
+
+topStage, topAnnStage, topSpliceStage :: ThStage
+topStage       = Comp
+topAnnStage    = Splice
+topSpliceStage = Splice
+
+instance Outputable ThStage where
+   ppr Splice        = text "Splice"
+   ppr Comp         = text "Comp"
+   ppr (Brack s _ _) = text "Brack" <> parens (ppr s)
+
 type ThLevel = Int     
-       -- Indicates how many levels of brackets we are inside
-       --      (always >= 0)
+        -- See Note [Template Haskell levels] in TcSplice
        -- Incremented when going inside a bracket,
        -- decremented when going inside a splice
        -- NB: ThLevel is one greater than the 'n' in Fig 2 of the
        --     original "Template meta-programming for Haskell" paper
 
-impLevel, topLevel :: ThLevel
-topLevel = 1   -- Things defined at top level of this module
+impLevel, outerLevel :: ThLevel
 impLevel = 0   -- Imported things; they can be used inside a top level splice
+outerLevel = 1 -- Things defined outside brackets
+-- NB: Things at level 0 are not *necessarily* imported.
+--     eg  $( \b -> ... )   here b is bound at level 0
 --
 -- For example: 
 --     f = ...
 --     g1 = $(map ...)         is OK
 --     g2 = $(f ...)           is not OK; because we havn't compiled f yet
 
-
-data ThStage
-  = Comp   ThLevel                     -- Ordinary compiling, usually at level topLevel but annotations use a lower level
-  | Splice ThLevel                     -- Inside a splice
-  | Brack  ThLevel                     -- Inside brackets; 
-          (TcRef [PendingSplice])      --   accumulate pending splices here
-          (TcRef LIE)                  --   and type constraints here
-topStage, topAnnStage, topSpliceStage :: ThStage
-topStage       = Comp topLevel
-topAnnStage    = Comp (topLevel - 1)
-topSpliceStage = Splice (topLevel - 1) -- Stage for the body of a top-level splice
+thLevel :: ThStage -> ThLevel
+thLevel Splice        = 0
+thLevel Comp          = 1
+thLevel (Brack s _ _) = thLevel s + 1
 
 ---------------------------
 -- Arrow-notation context