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digitalmars.D - type switch

reply Paul D. Anderson <paul.d.removethis.anderson comcast.andthis.net> writes:
I came across Lava, an experimental programming language ("What's hotter than
Java?") http://lavape.sourceforge.net/LavaHomePage.htm Lava has some curious
features (they claim to have no syntax, but the syntax is just hidden by the
"programming environment").

One of the evils of other programming languages, according to the website, is
the infamous type cast: "Type casts are not only ugly, nasty, and annoying but
much worse: their justification can only be assessed on the basis of a more
comprehensive understanding of the dynamic program behavior in general."

While I don't feel quite that strongly about type casts, one use case for
casting does bother me: Using a generic object as the type of a member, and
then specializing the object when you use it.

A good example is a Token type in a lexer. The token typically has a line and
column number, a string (the text of the token), a token type (an enumeration
or int) and a value. The value is an integer for an integer token, a string for
a string, an enum for a symbol or keyword, etc. So the type of the value field
is "object", and you use the token type to tell you how to cast the value field
when you use it.

The Lava solution to this problem is to allow an explicit type switch:

switch (value.type) {

case int:  // value is an int
    value += 3;
    break;

case string: // value is a string
    string sub = substring(value, 1, 5);
    break;

case symbol: // value is a symbol type
    int index = value.getSymbolIndex;
    break;

}

(Lava doesn't actually give an example of a type switch -- I just guessed at
what it would look like.)

Anyone else think this is a good idea?

Does anyone know of another language that does this? (I don't have much
knowledge of functional or logical languages.)

Paul
Sep 01 2009
parent Jarrett Billingsley <jarrett.billingsley gmail.com> writes:
On Tue, Sep 1, 2009 at 7:39 PM, Paul D.
Anderson<paul.d.removethis.anderson comcast.andthis.net> wrote:
 I came across Lava, an experimental programming language ("What's hotter =

urious features (they claim to have no syntax, but the syntax is just hidde= n by the "programming environment").
 One of the evils of other programming languages, according to the website=

ying but much worse: their justification can only be assessed on the basis = of a more comprehensive understanding of the dynamic program behavior in ge= neral."
 While I don't feel quite that strongly about type casts, one use case for=

d then specializing the object when you use it.
 A good example is a Token type in a lexer. The token typically has a line=

eration or int) and a value. The value is an integer for an integer token, = a string for a string, an enum for a symbol or keyword, etc. So the type of= the value field is "object", and you use the token type to tell you how to= cast the value field when you use it.
 The Lava solution to this problem is to allow an explicit type switch:

 switch (value.type) {

 case int: =A0// value is an int
 =A0 =A0value +=3D 3;
 =A0 =A0break;

 case string: // value is a string
 =A0 =A0string sub =3D substring(value, 1, 5);
 =A0 =A0break;

 case symbol: // value is a symbol type
 =A0 =A0int index =3D value.getSymbolIndex;
 =A0 =A0break;

 }

 (Lava doesn't actually give an example of a type switch -- I just guessed=

 Anyone else think this is a good idea?

 Does anyone know of another language that does this? (I don't have much k=

Just looks like simple pattern matching. It's very common in functional languages.
Sep 01 2009