Today, I made a typing mistake in the REPL and discovered a strange behaviour. Here's a sample of the interaction:
Welcome to Racket v6.11.
> (define x 3)
> x
3
> x'
3
> x
'x
>
So the first x I typed resulted in 3, which is expected. The x' I accidentally typed resulted in 3, which is unexpected. The last x resulted in 'x (!!).
It seems like there is something I don't understand about how the REPL reads values. Could someone explain why the REPL behaves in this way?
See Racketrivia: Using ' as an "identifier suffix" on the racket mailing list. The reply by Robby Findler and the reply by Matthias Felleisen both explain this.
The first line, x, is normal.
The second line, x', is actually an "expression and-a-half." It's interpreted as an expression x followed by an unfinished expression '. The unfinished expression is allowed to be finished on the next line. Whatever you put on the next line will be put as the second half of ' next-line.
That means the third x is actually interpreted as the second half of ' x.
You can see a better example of unfinished expressions with parentheses:
> 1 (list 2 ; 1 followed by an unfinished expression
1
> 3 4) ; finishing it
'(2 3 4)
> 3 4) ; on its own without the `(list 2` before it it's an error
3
4
; readline-input:13:3: read-syntax: unexpected `)` [,bt for context]
The quote after an expression is interpreted in a similar way:
> 1 ' ; 1 followed by an unfinished expression
1
> (indefatigable inexhaustible) ; finishes previously unfinished, put after a quote
'(indefatigable inexhaustible)
> (indefatigable inexhaustible) ; on its own without the quote before it it's an error
; indefatigable: undefined;
; cannot reference an identifier before its definition
; in module: top-level
; internal name: indefatigable
