ghci macros override built-ins for command expansion
I have a ghci macro
:tsu from the ghc-vis package, which I installed a long time ago. In HEAD ghci (since the patch for #8113 (closed)) this causes
:t to expand to
:tsu, rather than
:type. That happened to result in a weird error the first time I tried to use
:t (something like
Prelude.read: no parse), and it took me a while to diagnose that my
.ghci file was the issue!
I don't like this new behavior because it forces me to either change my ghci habits (start using
:type instead of
:t) or avoid macros starting with any letter that I currently use as a single-letter ghci command. I set this ticket priority to highest because in any event this new behavior shouldn't sneak in to a GHC release unnoticed.
Below is my proposal for how
:commands should be interpreted now that built-in commands can be overridden (#8113 (closed)), copied from a comment I made recently on that ticket.
I suppose what I specifically want to happen when I enter a
:command is an algorithm like this.
If the name I entered is an exact match for a macro or built-in, use that name.
Otherwise, try to complete the name to the name of a built-in in the traditional way. If this succeeds, use the resulting name.
Otherwise, try to complete the name to the name of a macro, and use the resulting name if that succeeds, otherwise give up.
In all cases where we got a name, use the macro of that name if there is one, and otherwise use the built-in. (Obviously, for
::command, ignore macros entirely.)
In other words, built-ins should take precedence over macros for the purpose of name completion, but macros should take precedence over built-ins for the purpose of name lookup. This is backwards-compatible from the perspective of the user who is not aware of the change—
:t will always mean
:type, as long as the user has no macro named
:t, just like in previous versions of ghci—while still allowing the aware user to redefine exactly what
:type means. And it's flexible enough in that if the user really wants
:t to complete to some other macro
:test that they've written, they can always define another macro
:t to expand to