inclarity in threaded lexer docs leads to confusing failure

[xplat]

https://www.haskell.org/happy/doc/html/sec-monads.html#sec-lexers states that

is the name of the lexical analyser function, and is a token that is to be treated as the end of file.

I had created a token named 'eof' under %token and set to eof. This seemed to work, but when I finished the rest of the grammar to a compiling state, it would immediately fail at the first token. This is because eof was taken as a variable pattern that matched everything, so everything was considered an end of file. The documentation could stand to be louder that this is a Haskell pattern matching the token type, and not the name of a token.

Cc: @cartazio

[cartazio]

@xplat https://github.com/haskell/happy/blob/master/examples/glr/expr-monad/Expr.y does this example illustrate that?

[cartazio]

So the second thing will be used In pattern matching position so it needs to be a constructor or pattern synonym (I’m assuming a pattern synonym is possible?)

[xplat]

Yes

happy/examples/glr/expr-monad/Expr.y

Line 9 in 1e2493c

%lexer { lexer } { TokenEOF }

is how it should actually be done ...