CODEWITHSUNDEEP
schemeracket
user618815
user618815

Reputation:

How to match , in match in Racket?

if I have something like this (define s (hi,there)) then how can I write in match like (match s [(,h , ,t)] ...) But it is not working, because match needs the , so how can I do this?

Upvotes: 6

Views: 1880

Answers (3)

Daniel
Daniel

Reputation: 6755

I think you might be confused about where you need commas. In Racket, you do not use commas to separate elements in a list. Instead, you just use whitespace. Tell me if this is wrong, but what I imagine is that you are trying to match an expression like (define s '(hi there)). To do that, you would use

(match s
  [`(,h ,t) ...])

Then, in the area where the elipses is, the variable h has the value 'hi, and the variable t has the value 'there

Upvotes: 1

Dan Burton
Dan Burton

Reputation: 53665

Use a backslash if you want to use comma as a symbol inside of a quoted section:

> (define s '(hi \, there))
> (match s [(list h c t) (symbol->string c)])
","

And use '|,| for the standalone comma symbol.

> (match s [(list h '|,| t) (list h t)])
'(hi there)

In either case, you really should use whitespace to separate things, and use lists.

(define s (hi,there)) is not valid Racket.

Upvotes: 2

soegaard
soegaard

Reputation: 31147

First note that the comma , is a special reader abbreviation. The (hi,there) is a read as (hi (unquote there)). This is difficult to spot - since the default printer prints lists whose first element is an unquote in a special way.

Welcome to DrRacket, version 5.3.0.14--2012-07-24(f8f24ff2/d) [3m].
Language: racket.
> (list 'hi (list 'unquote 'there))
'(hi ,there)

Therefore the pattern you need is '(list h (list 'unquote t))'.

> (define s '(hi,there))
> (match s [(list h (list 'unquote t)) (list h t)])
(list 'hi 'there)

Upvotes: 7

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