I just wondered whether it's possible to match against the same values for multiple times with the pattern matching facilities of functional programming languages (Haskell/F#/Caml).
Just think of the following example:
plus a a = 2 * a plus a b = a + b
The first variant would be called when the function is invoked with two similar values (which would be stored in a
).
A more useful application would be this (simplifying an AST).
simplify (Add a a) = Mult 2 a
But Haskell rejects these codes and warns me of conflicting definitions for a
- I have to do explicit case/if-checks instead to find out whether the function got identical values. Is there any trick to indicate that a variable I want to match against will occur multiple times?
This is called a nonlinear pattern. There have been several threads on the haskell-cafe mailing list about this, not long ago. Here are two:
http://www.mail-archive.com/[email protected]/msg59617.html
http://www.mail-archive.com/[email protected]/msg62491.html
Bottom line: it's not impossible to implement, but was decided against for sake of simplicity.
By the way, you do not need if
or case
to work around this; the (slightly) cleaner way is to use a guard:
a `plus` b | a == b = 2*a | otherwise = a+b
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