#13124: Template Haskell: Floating point literals that cannot be represented by rationals -------------------------------------+------------------------------------- Reporter: lerkok | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Template Haskell | 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: | -------------------------------------+------------------------------------- Comment (by lerkok): @goldfire That is a good point! But if TH aims at being able to produce constants of floats as a valid splice, then this will be an issue. Imagine a use case where someone uses template-haskell to "optimize" or "stabilize" numeric expressions. For instance, something like Herbie: http://herbie.uwplse.org/. There's already a GHC plugin for that: https://github.com/mikeizbicki/HerbiePlugin. One can easily imagine doing a similar transformation via TH, and having access to arbitrary floating constants would be useful. There would be no easy way to write that in TH today as all the "magic" constants would have to be converted to rationals, thus having anomalies. The other place this came up is in the FloatingHex library: https://hackage.haskell.org/package/FloatingHex-0.4. Currently this isn't a big issue there since literals supported are typically only positive ones, but one can imagine having a similar quasi-quoter that supported negative literals as well. Unfortunately, you cannot represent numbers such as "-0.0" (negative-zero!), as there is no way to write that as a rational number. (Note that negative-zero is a "valid" float, which can be told apart from the good old positive-zero.) I wouldn't go as far as saying that this is a huge shortcoming, but I don't see why TH can't just use a good old double/float there instead of requiring a Rational? Is there a good reason for requiring a Rational there? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13124#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler