CODEWITHSUNDEEP
f#pattern-matching
Mads Andersen
Mads Andersen

Reputation: 3153

Simplify expression F#

I want the expression (x-x) to be simplified to 0.

type aexpr = 
| CstI of int
| Var of string
| Add of aexpr * aexpr
| Sub of aexpr * aexpr
| Mul of aexpr * aexpr;;

let rec simplify expr =
match expr with
| Add(CstI(n1), CstI(n2)) ->CstI(n1 + n2)
| Sub(CstI(n1), CstI(n2)) ->CstI(n1 - n2)
| Mul(CstI(n1), CstI(n2)) ->CstI(n1 * n2)
| Add(e, CstI(0)) -> simplify e
| Add(CstI(0), e) -> simplify e
| Sub(CstI(0), e) -> simplify e
| Sub(e, CstI(0)) -> simplify e
| Sub(Var(x1), Var(x2)) when x1.Equals(x2) -> CstI(0) // <- not working
| Mul(CstI(0), e) -> CstI(0)
| Mul(e, CstI(0)) -> CstI(0)
| Mul(CstI(1), e) -> simplify e
| Mul(e, CstI(1)) -> simplify e
| _ -> expr;;

This does not do it. I do not see what I am doing wrong. Hope you can help me :)

Edit: It compiles fine but it does not do anything. Ie.

let e = Mul(CstI(1), Add(CstI(4), Sub(Var("x"), Var("x"))));;

In f# interactive:

let result = simplify e;;
val result : aexpr = Add (CstI 4,Sub (Var "x",Var "x"))

Result should be CstI 4

Upvotes: 2

Views: 631

Answers (2)

sepp2k
sepp2k

Reputation: 370112

simplify (Sub (Var "x", Var "x")) works just fine. The reason that your example does not work is that simplify does not traverse the whole tree, so the Sub (Var "x", Var "x") part of the tree is never simplified.

In short, you're missing a case Sub (e,e) -> Sub (simplify e, simplify e) and the same for the other operators.

Upvotes: 6

Daniel Pratt
Daniel Pratt

Reputation: 12077

It seems to work here. You're not dealing with a case-sensitivity issue with the string comparison, are you?

Upvotes: 0

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