Parse error in pattern in haskell

Question

When implementing the DFS algorithm, I got to use the Imperative features. Here is the code

type ST s a = s -> (a, s)

returnST :: a -> ST s a
returnST a s = (a, s)

thenST :: ST s a -> (a -> ST s b) -> ST s b
(m 'thenST' k) s = k a t where (a, t) = m s

But the GHCi 6.12.3 give me a 'Parse error in pattern' in the last line of code. I get this piece of code from an paper. Is there any possibility that the complier used by the author of the paper got no strict grammar rules as now? And how to fix this error?

You know that DFS doesn't require mutable state, right? Your first sentence implies otherwise. — Thomas M. DuBuisson
– Thomas M. DuBuisson, Commented May 2, 2011 at 17:01

Fred Foo · Accepted Answer · 2011-05-02 13:31:48Z

6

Change

'thenST'

to

`thenST`

The backticks turn an ordinary function name into an infix operator. They may have been turned into single quotes by an editor, word processor or document viewer.

answered May 2, 2011 at 13:31

Fred Foo

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2 Comments

machinarium Over a year ago

Sorry, I made a mistake in the question. In my code, it's thenST. So the problem has nothing related with backticks

Fred Foo Over a year ago

@machinarium: then I can't help you any further. GHCi 6.12.1 loads the code you posted just fine when I insert backticks.

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