#12874: Read/Show Incompatibility in base -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * component: Compiler => libraries/base Comment: This does indeed seem to be a problem with `Proxy`'s `Read` instance. I've copied its current implementation below: {{{#!hs instance Read (Proxy s) where readsPrec d = readParen (d > 10) (\r -> [(Proxy, s) | ("Proxy",s) <- lex r ]) }}} That `readParen (d > 10)` is a problem, since if the precendence `d` is greater than 10, then in order for parsing to succeed, `"Proxy"` must be surrounded by at least one set of parentheses. But, as you noted, this is too stringent of a requirement, since `show (Thing Proxy)` yields `"Thing Proxy"`, without any parentheses. One way to fix this is to ignore the precedence: {{{#!hs instance Read (Proxy s) where readsPrec _ = readParen False (\r -> [(Proxy, s) | ("Proxy",s) <- lex r ]) }}} Then `"Thing Proxy"`, `"Thing (Proxy)"`, etc. all parse. But then again, this definition is basically equivalent to what `deriving Read` produces. Why not use that? FWIW, `Proxy` isn't the only type to suffer from this problem. It looks like `Coercion`, `:~:`, `:~~:`, and `U1` also have `Read` instances with the same issue. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12874#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#12874: Read/Show Incompatibility in base -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: libraries/base | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3871 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ryan Scott <ryan.gl.scott@…>): In [changeset:"8fd959998e900dffdb7f752fcd42df7aaedeae6e/ghc" 8fd95999/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="8fd959998e900dffdb7f752fcd42df7aaedeae6e" Make the Read instance for Proxy (and friends) ignore precedence Summary: The `Read` instance for `Proxy`, as well as a handful of other data types in `base` which only have a single constructor, are doing something skeevy: they're requiring that they be surrounded by parentheses if the parsing precedence is sufficiently high. This means that `"Thing (Proxy)"` would parse, but not `"Thing Proxy"`. But the latter really ought to parse, since there's no need to surround a single constructor with parentheses. Indeed, that's the output of `show (Thing Proxy)`, so the current `Read` instance for `Proxy` violates `read . show = id`. The simple solution is to change `readParen (d > 10)` to `readParen False` in the `Read` instance for `Proxy`. But given that a derived `Read` instance would essentially accomplish the same thing, but with even fewer characters, I've opted to just replace the hand-rolled `Read` instance with a derived one. Test Plan: make test TEST=T12874 Reviewers: ekmett, austin, hvr, goldfire, bgamari Reviewed By: bgamari Subscribers: rwbarton, thomie GHC Trac Issues: #12874 Differential Revision: https://phabricator.haskell.org/D3871 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12874#comment:3> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler