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Execute HQ9+: Difference between revisions
→{{header|Ela}}
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===Standard approach===
<lang ela>open
where eval' [] = ()
eval' (x::xs) | be 'H' = h! $ eval' xs
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| be '+' = p! $ eval' xs
| else = fail ("Unrecognized " ++ x)
where
where bottles [] = ()
bottles (x::xs) = rec write
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An interpreter itself has no side effects:
<lang ela>open
where eval' [] a = []
eval' (x::xs) a | be 'H' = h :: eval' xs a
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| be '9' = force n :: eval' xs a
| be '+' = eval' xs (a+1)
| else = fail "Invalid instruction."
where be c = upper x == c
where bottles [] = ""
bottles (x::xs) =
show x ++ " bottles of beer of the wall\r\n"
++ show x ++ " bottles of beer\r\n"
++ "Take one down, pass it around\r\n"
++ bottles xs</lang>
It slightly alters an original HQ9+ specification. HQ9+ is an impure language that does console output. However console output is the only interaction that a user can see when executing HQ9+ program. This interpreter doesn't output to console but instead generates a list with all outputs. An accumulator is moved to the interpter arguments and the need for a reference cell is eliminated. Once an interpreter completes a client code can output to console like so:
<lang ela>
each writen <| eval "HQ9+"</lang>
=={{header|Erlang}}==
| |||