Simon Peyton Jones pushed to branch wip/az/ghc-10.1-exactprint-update-wip at Glasgow Haskell Compiler / GHC Commits: 2d4151e9 by Simon Peyton Jones at 2026-05-28T00:10:26+01:00 Added an intro section - - - - - 1 changed file: - ExactPrint.md Changes: ===================================== ExactPrint.md ===================================== @@ -13,6 +13,67 @@ information directly into every AST node to allow the tree to be reprinted _exac original source, byte-for-byte (modulo tab expansion). When a tool modifies the AST it adjusts only the annotations it needs to change; all surrounding nodes reprint themselves unchanged. +## Background: the syntax tree + +The AST of the source program is represented using the data types defined in `Language.Haskell.Syntax.*`. +This data type uses the "Trees That Grow (TTG)" plan; +see [Implementing trees that grow](https://gitlab.haskell.org/ghc/ghc/-/wikis/implementing-trees-that-grow) + +For example, in (GHC-independent) `Language.Haskell.Syntax.Expr`: +``` +data HsExpr p + = ... many constructors including ... + | HsLet (XLet p) + (HsLocalBinds p) + (LHsExpr p) + +type family XLet p +type LHsExpr p = XRec p (HsExpr p) + +``` +Note that +* The `(XLet p)` field is the *extension field* of the `HsLet` data constructor, + where `XLet` is a type family. +* Almost every node is wrapped in an `XRec`, another type family. That makes it easy + for clients to attach arbitrary information to each node. + +GHC specialises this data a type in `GHC.Hs.*`, as follows: +``` +data Pass = Parsed | Renamed | Typechecked + +data GhcPass (c :: Pass) where + GhcPs :: GhcPass 'Parsed + GhcRn :: GhcPass 'Renamed + GhcTc :: GhcPass 'Typechecked + +type family XRec p a = r | r -> a +type instance XRec (GhcPass p) t = XRecGhc t + +-- (XRecGhc tree) wraps `tree` in a GHC-specific, +-- but pass-independent, source location +type XRecGhc t = GenLocated (Anno t) t + +data GenLocated l e = L l e +type family Anno t +``` +So, via `XRec`, every node `e :: t` in the GHC-specific version of HsSyn is wrapped in +a `L ann e`, where `ann :: Anno t` is the annotation on the node. + +Notice that there are two places we can hang information: +* (EXT) **Constructor-specific punctuation**: the extension field of each data constructor + can contain information that is specific to that constructor. Example: the location + of the keywords `let` and `in` for the `HsLet` construct. + +* (XREC) **Entire-node information**: the `Anno t` field that wraps almost every node in the + syntax tree can contain information that is needed for *every* node. Classic example: + the `SrcSpan` of the node. + +**NOTE**: currently there is information in (XREC) that more properly belongs in (EXT). +A refactoring project is under way to put this right. + + +## File layout + The subsystem lives primarily in `utils/check-exact/` and is exposed through three main layers: | Layer | Key operation | Purpose | View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/2d4151e92b1cce8fcb7140619d008ac3... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/2d4151e92b1cce8fcb7140619d008ac3... You're receiving this email because of your account on gitlab.haskell.org.