Determine type of a variable in Tcl

Question

I'm looking for a way to find the type of a variable in Tcl. For example if I have the variable $a and I want to know whether it is an integer.

I have been using the following so far:

    if {[string is boolean $a]} {
    #do something
    }

and this seems to work great for the following types:
alnum, alpha, ascii, boolean, control, digit, double, false, graph, integer, lower, print, punct, space, true, upper, wordchar, xdigit

However it is not capable to tell me if my variable might be an array, a list or a dictionary. Does anyone know of a way to tell if a variable is either of those three?

Answer 1

Tcl's variables don't have types (except for whether or not they're really an associative array of variables — i.e., using the $foo(bar) syntax — for which you use array exists) but Tcl's values do. Well, somewhat. Tcl can mutate values between different types as it sees fit and does not expose this information[*]; all you can really do is check whether a value conforms to a particular type.

Such conformance checks are done with string is (where you need the -strict option, for ugly historical reasons):

if {[string is integer -strict $foo]} {
    puts "$foo is an integer!"
}

if {[string is list $foo]} {    # Only [string is] where -strict has no effect
    puts "$foo is a list! (length: [llength $foo])"
    if {[llength $foo]&1 == 0} {
        # All dictionaries conform to lists with even length
        puts "$foo is a dictionary! (entries: [dict size $foo])"
    }
}

Note that all values conform to the type of strings; Tcl's values are always serializable.

[EDIT from comments]: For JSON serialization, it's possible to use dirty hacks to produce a “correct” serialization (strictly, putting everything in a string would be correct from Tcl's perspective but that's not precisely helpful to other languages) with Tcl 8.6. The code to do this, originally posted on Rosetta Code is:

package require Tcl 8.6

proc tcl2json value {
    # Guess the type of the value; deep *UNSUPPORTED* magic!
    regexp {^value is a (.*?) with a refcount} \
        [::tcl::unsupported::representation $value] -> type

    switch $type {
        string {
            # Skip to the mapping code at the bottom
        }
        dict {
            set result "{"
            set pfx ""
            dict for {k v} $value {
                append result $pfx [tcl2json $k] ": " [tcl2json $v]
                set pfx ", "
            }
            return [append result "}"]
        }
        list {
            set result "\["
            set pfx ""
            foreach v $value {
                append result $pfx [tcl2json $v]
                set pfx ", "
            }
            return [append result "\]"]
        }
        int - double {
            return [expr {$value}]
        }
        booleanString {
            return [expr {$value ? "true" : "false"}]
        }
        default {
            # Some other type; do some guessing...
            if {$value eq "null"} {
                # Tcl has *no* null value at all; empty strings are semantically
                # different and absent variables aren't values. So cheat!
                return $value
            } elseif {[string is integer -strict $value]} {
                return [expr {$value}]
            } elseif {[string is double -strict $value]} {
                return [expr {$value}]
            } elseif {[string is boolean -strict $value]} {
                return [expr {$value ? "true" : "false"}]
            }
        }
    }

    # For simplicity, all "bad" characters are mapped to \u... substitutions
    set mapped [subst -novariables [regsub -all {[][\u0000-\u001f\\""]} \
        $value {[format "\\\\u%04x" [scan {& } %c]]}]]
    return "\"$mapped\""
}

Warning: The above code is not supported. It depends on dirty hacks. It's liable to break without warning. (But it does work. Porting to Tcl 8.5 would require a tiny C extension to read out the type annotations.)

---

[*] Strictly, it does provide an unsupported interface for discovering the current type annotation of a value in 8.6 — as part of ::tcl::unsupported::representation — but that information is in a deliberately human-readable form and subject to change without announcement. It's for debugging, not code. Also, Tcl uses rather a lot of different types internally (e.g., cached command and variable names) that you won't want to probe for under normal circumstances; things are rather complex under the hood…

Answer 2

The other answers all provide very useful information, but it's worth noting something that a lot of people don't seem to grok at first.

In Tcl, values don't have a type... they question is whether they can be used as a given type. You can think about it this way

string is integer $a

You're not asking

Is the value in $a an integer

What you are asking is

Can I use the value in $a as an integer

Its useful to consider the difference between the two questions when you're thinking along the lines of "is this an integer". Every integer is also a valid list (of one element)... so it can be used as either and both string is commands will return true (as will several others for an integer).