Funny thing. It was just last Friday that Joseph Papp, producer of the Broadway musical "A Chorus Line," was complaining about Ronald Reagan's cuts in arts funding, and yet, there he was yesterday, tapping his black boot to music on the East Room's parquet floor.

"Yes, it is ironic," he said, not at all unplesantly, as several pairs of legwarmers leapt and thumped from above. He was watching, as was a gaggle of White House staffers loitering in the East Room doorway, an afternoon rehearsal of highlights from "A Chorus Line." It was the main entertainment at last night's black-tie dinner for the nation's governors.

The Reagan's who like big, splashy musicals, asked Papp several weeks ago to come down to perform at the White House. "A Chorus Line," might be more bittersweet than splashy, but Papp said sure. "For them," he said, meaning the 26 cast members who've performed this show hundreds and hundreds of times, "it's an extra surge."

Certainly, performing in the East Room had its own glamor. The stage was tiny and the lights impromptu, but then, Dolley Madison watched in oil from a wall. Not only that, a dangling crystal ball from one of the room's chandeliers was hanging so low as to hit the dancers in their heads. Atmospheric, but hazardous. Somebody removed it.

As it was, the White House became just icing on the petit four the closing course after chateaubriand bearnaise, salmon, tomatoes florentine, etc., etc.) for the dancers last night. That's because the five-year-running Broadway show has won the Pulitizer Prize, Drama Critics Award and Tony Award. And just recently, the cast performed for the ex-hostages.

Yesterday, Papp listened to the rehearsal of "One," the closing number, with a content smile on his face. He tapped his boot some more, perused a White House schedule for the cast reminding them that Washington bars close at "2 a.m. sharp," and announced, decorously, that he thought it wasn't all proper to bring up the nasty subject of budget cuts at the black-tie dinner.

Oh? Pap's public theater ventures get more than $300,000 from the National Endowment for the Arts, and if Reagan cuts that budget 50 percent as he says he will . . .

"There's Nancy Reagan," Papp responded. "I may mention something to her."

More than 100 guests were to watch the show, the bulk of them governors. They're in town for their annual winter conference that is traditionally finished off with the exotic flora (tulips, everywhere) and local fauna (Chief of Staff Jim Baker, presidential counselors Edwin Meese, other supporting characters) of a White House dinner.

For Reagan, it's a chance to be nice to Democratic as well as Republican governors and so enhance his statesmanship; for the governors, it's a chance to remind the president, especially this president, that they really need that water project.

And of course, a few people really do like to eat. And dance.