[Haskell-cafe] Template Haskell question

[Simon Peyton Jones]

If you think there's a bug, you may want to file a GHC ticket about this, and include the results of your digging:
https://gitlab.haskell.org/ghc/ghc/-/issues

Thanks,

Simon

|  -----Original Message-----
|  From: Haskell-Cafe <haskell-cafe-bounces at haskell.org> On Behalf Of
|  ????
|  Sent: 28 April 2021 09:43
|  To: Haskell Cafe <haskell-cafe at haskell.org>
|  Subject: Re: [Haskell-cafe] Template Haskell question
|  
|  I've been digging through GHC source code to understand the reasons
|  behind these behaviors.
|  
|  1. There are use-cases of TemplateHaskell which do not want duplicate
|  variables in patterns to be error. See a note in source code:
|  
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|  
|  Quoting the linked note,
|  
|  > Consider this TH term construction:
|  >   do { x1 <- TH.newName "x"   -- newName :: String -> Q TH.Name
|  >      ; x2 <- TH.newName "x"   -- Builds a NameU
|  >      ; x3 <- TH.newName "x"
|  >
|  >      ; let x = mkName "x"     -- mkName :: String -> TH.Name
|  >                               -- Builds a NameS
|  >
|  >      ; return (LamE (..pattern [x1,x2]..) $
|  >                LamE (VarPat x3) $
|  >                ..tuple (x1,x2,x3,x)) }
|  >
|  > It represents the term   \[x1,x2]. \x3. (x1,x2,x3,x)
|  >
|  > a) We don't want to complain about "x" being bound twice in
|  >    the pattern [x1,x2]
|  > b) We don't want x3 to shadow the x1,x2
|  > c) We *do* want 'x' (dynamically bound with mkName) to bind
|  >    to the innermost binding of "x", namely x3.
|  > d) When pretty printing, we want to print a unique with x1,x2
|  >    etc, else they'll all print as "x" which isn't very helpful
|  
|  To do this, a variable name has one of these two flavors:
|  "system" flavor for names generated by `newName`, and "normal" flavor
|  for names made by `mkName`. Duplicate variable name error is not
|  checked for "system" flavored names.
|  
|  Actually, using `mkName` works in an intended way (cause error)
|  
|  ghci> (\x $(varP (mkName "x")) -> x) 1 2
|  
|  <interactive>:10:3: error:
|      • Conflicting definitions for ‘x’
|        Bound at: <interactive>:10:3
|                  <interactive>:10:7-23
|      • In a lambda abstraction
|  
|  2. Quoting brackets ([| ... |], [p| ... |]) are translated to
|  TemplateHaskell code (in Q Monad) by desugarar. The desugarar
|  represents *every* bound variable name by names generated by
|  TH.newName. For example, the desugared code of [| \x y -> x y y |]
|  looks like below:
|  
|      do { x <- newName "x"; y <- newName "y";
|          return $ (LamE [VarP x, VarP y] (...) ) }
|  
|  And [p| x |] is translated in this manner too, causing the discussed
|  behavior.
|  
|  --
|  /* Koji Miyazato <viercc at gmail.com> */
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