#15661: Nullary constraint in GHCi breaks `:t` command -------------------------------------+------------------------------------- Reporter: taktoa | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: GHCi | Version: 8.4.3 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 simonpj): This behaviour is just what you'd expect. Remember: `:type <expression>` takes an ''arbitrary expression'' (not just a variable), finds its most general type, and shows that type. Here the expression happens to be just a single variable `x`. Fine, so we instantiate `x`, which gives rise to a "wanted" `(Show Foo)`. Then we attempt to solve and generalise, but `(Show Foo)` is not soluble. Hence the error. If you just want to see `x's` type, use `:info`, thus {{{ ghci> :info x x :: Show Foo => () -- Defined at <interactive>:3:26 }}} I think everything seems fine here. But it clearly wasn't what you expected. Does my explanation help? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15661#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler