I checked ghc source file, and found stdin input is processed by
runGHIC -> runGHCiInput -> ghciCompleteWord -> lookupCompletion -> lookupCommand'
At the end of lookupCommand
-- first, look for exact match (while preferring macros); then, look
-- for first prefix match (preferring builtins), *unless* a macro
-- overrides the builtin; see #8305 for motivation
return $ lookupExact str xcmds <|>
lookupExact str ghci_cmds <|>
(builtinPfxMatch >>= \c -> lookupExact (cmdName c) xcmds) <|>
builtinPfxMatch <|>
lookupPrefix str xcmds
Basically, it first lookup exact match, then prefix match in order. So for normal :xxx command, it follows ghci_cmds order, which is availableCommands in GhciSettings, i.e.
("?", keepGoing help, noCompletion),
("add", keepGoingPaths addModule, completeFilename),
("abandon", keepGoing abandonCmd, noCompletion),
("break", keepGoing breakCmd, completeIdentifier),
("back", keepGoing backCmd, noCompletion),
("browse", keepGoing' (browseCmd False), completeModule),
("browse!", keepGoing' (browseCmd True), completeModule),
("cd", keepGoing' changeDirectory, completeFilename),
("check", keepGoing' checkModule, completeHomeModule),
("continue", keepGoing continueCmd, noCompletion),
("cmd", keepGoing cmdCmd, completeExpression),
("ctags", keepGoing createCTagsWithLineNumbersCmd, completeFilename),
("ctags!", keepGoing createCTagsWithRegExesCmd, completeFilename),
("def", keepGoing (defineMacro False), completeExpression),
("def!", keepGoing (defineMacro True), completeExpression),
("delete", keepGoing deleteCmd, noCompletion),
("edit", keepGoing' editFile, completeFilename),
("etags", keepGoing createETagsFileCmd, completeFilename),
("force", keepGoing forceCmd, completeExpression),
("forward", keepGoing forwardCmd, noCompletion),
("help", keepGoing help, noCompletion),
("history", keepGoing historyCmd, noCompletion),
("info", keepGoing' (info False), completeIdentifier),
("info!", keepGoing' (info True), completeIdentifier),
("issafe", keepGoing' isSafeCmd, completeModule),
("kind", keepGoing' (kindOfType False), completeIdentifier),
("kind!", keepGoing' (kindOfType True), completeIdentifier),
("load", keepGoingPaths loadModule_, completeHomeModuleOrFile),
("load!", keepGoingPaths loadModuleDefer, completeHomeModuleOrFile),
("list", keepGoing' listCmd, noCompletion),
("module", keepGoing moduleCmd, completeSetModule),
("main", keepGoing runMain, completeFilename),
("print", keepGoing printCmd, completeExpression),
("quit", quit, noCompletion),
("reload", keepGoing' reloadModule, noCompletion),
("reload!", keepGoing' reloadModuleDefer, noCompletion),
("run", keepGoing runRun, completeFilename),
("script", keepGoing' scriptCmd, completeFilename),
("set", keepGoing setCmd, completeSetOptions),
("seti", keepGoing setiCmd, completeSeti),
("show", keepGoing showCmd, completeShowOptions),
("showi", keepGoing showiCmd, completeShowiOptions),
("sprint", keepGoing sprintCmd, completeExpression),
("step", keepGoing stepCmd, completeIdentifier),
("steplocal", keepGoing stepLocalCmd, completeIdentifier),
("stepmodule",keepGoing stepModuleCmd, completeIdentifier),
("type", keepGoing' typeOfExpr, completeExpression),
("trace", keepGoing traceCmd, completeExpression),
("undef", keepGoing undefineMacro, completeMacro),
("unset", keepGoing unsetOptions, completeSetOptions),
("where", keepGoing whereCmd, noCompletion)
so :c means :cd, :d means :def, follow the order.