pattern matching with same variable

Question

I have a doubt about the pattern matching in SML. In the book "Programming Language Pragmatics (Second edition)" I read that in SML when you use the pattern matching in a function, the same variable can not appear more than once. The explanation is that pattern matching is used to check if the function can be found in a defined pattern, and not to check if variables are in relation to each others. But it has confused me: why something like this could not be implemented? Here is an example:

fun Eq nil = false
| Eq (e) = false
| Eq x::x::resto = true //Error
| Eq x::y::resto = false

This snippet of code check if the first two elements of a list are equal. So, why the third line of the code is wrong? Thank you.

It can be implemented, but it's not in SML (for reasons I'm not aware of). There are languages that allow what you want, for example Erlang. gist.github.com/igstan/ac1d10e6d1b575278fd6e2a3b65ce437 — Ionuț G. Stan
– Ionuț G. Stan, Commented May 16, 2017 at 21:56

Alejandro C. · Accepted Answer · 2017-05-16 16:14:12Z

2

Pattern matching only compares compile-time constants and does simple variable assignment otherwise. You can't do a variable comparison via pattern matching. You need to remove the third line and extend the 4th line to

Eq x::y::resto = x = y

Pattern matching also happens in order. Since Eq e actually matches everything, it should be your final clause.

answered May 16, 2017 at 16:14

Alejandro C.

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1 Comment

Marco Luzzara Over a year ago

Thank you for the answer. instead of Eq e, it should have been Eq [e].

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