CODEWITHSUNDEEP
ocaml
user1514567
user1514567

Reputation: 91

Ocaml printing one statement after another

I am very new to Ocaml and ML in general and I have been having a very fundamental issue. I am using a pattern match and within one match I would like to print two or more concatenated statements. Eg. chan^"("^var^")"^op2^(poc p); chan^"("^var^")"^op^(poc p)

let processoperatorchange2 t2s2 proc2 op op2=
  let rec poc2 p = match p with
    | Zero ->  "0"
    | Pproc (x) -> String.lowercase x
    | In(chan, var, _, p, _) ->  chan^"("^var^")"^op^(poc2 p); chan^"("^var^")"^op2^(poc2 p)
  in poc2 proc2

But then each time I run this, the only statement printed is the last one after the semi colon. Can I get some help with this?

Upvotes: 0

Views: 409

Answers (1)

ivg
ivg

Reputation: 35210

Your function does not print a statement but builds a string, thus it returns a value, and doesn't perform any side-effects. The semicolon operator, when interspersed between two expressions, doesn't combine the value produced from these expressions, thus if you have "hello"; "world" the result is "world". That is what happens in your case when you do

chan^"("^var^")"^op^(poc2 p); chan^"("^var^")"^op2^(poc2 p)

Everything on the lift is just thrown away.

A quick fix would be to concatenate them, e.g.,

chan^"("^var^")"^op^(poc2 p) ^ ";\n" ^ chan^"("^var^")"^op2^(poc2 p)

But in general, an idiomatic way to print AST is to use the Format module, and implement a recursive pp function, that has type Format.formatter -> 'a -> unit. Note the return type, the function doesn't build a string (that is usually an operation of quadratic complexity), but rather prints it into generic output stream.

Upvotes: 2

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