Basic Examples

In[]:=

Initialize a symbol:

○
SetUnless
[x,1,IntegerQ]
In[]:=
1
Out[]=

The value is not set if it already satisfies the given criteria:

○
SetUnless
[x,2,IntegerQ]
In[]:=
1
Out[]=
x
In[]:=
1
Out[]=

The
expr
does not need to be a symbol:

○
SetUnless
[f[1],Pi,NumericQ]
In[]:=
π
Out[]=
○
SetUnless
[f[1],E,NumericQ]
In[]:=
π
Out[]=
f[1]
In[]:=
π
Out[]=

Use the operator forms:

○
SetUnless
[StringQ][a,"hello"]
In[]:=
hello
Out[]=
○
SetUnless
[StringQ][a,"world"]
In[]:=
hello
Out[]=
a
In[]:=
hello
Out[]=
○
SetUnless
[123,IntegerQ][b]
In[]:=
123
Out[]=
○
SetUnless
[456,IntegerQ][b]
In[]:=
123
Out[]=
b
In[]:=
123
Out[]=
○
SetUnless
[ListQ][{1,2,3}][c]
In[]:=
{1,2,3}
Out[]=
○
SetUnless
[ListQ][{4,5,6}][c]
In[]:=
{1,2,3}
Out[]=
c
In[]:=
{1,2,3}
Out[]=

Scope

In[]:=

Use
Unevaluated
to prevent evaluation of the second argument unless needed:

file=FileNameJoin@{$TemporaryDirectory,CreateUUID[]}
In[]:=
C:\Users\rhennigan\AppData\Local\Temp\0141d862-44cf-4bdc-b288-aedcaa17e601
Out[]=
○
SetUnless
[file,Unevaluated[Echo[CreateFile[]]],FileExistsQ]
In[]:=
C:\Users\rhennigan\AppData\Local\Temp\m-08122de9-5bdd-4e0e-8053-4e64265fca3f
»
C:\Users\rhennigan\AppData\Local\Temp\m-08122de9-5bdd-4e0e-8053-4e64265fca3f
Out[]=
○
SetUnless
[file,Unevaluated[Echo[CreateFile[]]],FileExistsQ]
In[]:=
C:\Users\rhennigan\AppData\Local\Temp\m-08122de9-5bdd-4e0e-8053-4e64265fca3f
Out[]=

Without using
Unevaluated
, the second argument will evaluate, even if not being used:

○
SetUnless
[file,Echo[CreateFile[]],FileExistsQ]
In[]:=
C:\Users\rhennigan\AppData\Local\Temp\m-4e6e65f2-8bc7-433f-86c6-f284d5306cf1
»
C:\Users\rhennigan\AppData\Local\Temp\m-08122de9-5bdd-4e0e-8053-4e64265fca3f
Out[]=

Use an additional
Unevaluated
to get the same behavior as
SetDelayed
:

○
SetUnless
[x,Unevaluated[Unevaluated[Echo["evaluated"]]],False&]
In[]:=
Definition[x]
In[]:=
x:=Echo[evaluated]
Out[]=

Applications

In[]:=

Initialize a counter if it doesn’t already exist before incrementing:

Do
○
SetUnless
[counter,0,IntegerQ];counter++,10;​​counter
In[]:=
10
Out[]=

If the counter ends up with an invalid value,
Increment
will no longer return an integer:

counter="bad value"
In[]:=
bad value
Out[]=
counter++
In[]:=
bad value
Out[]=

Use
SetUnless
to ensure a valid value before incrementing:

Create a function that stores data in an association and initializes it if necessary:

Check the data:

Clear the data:

Now save will reinitialize the association and start again:

Saving will reset the association if it is no longer valid:

Properties and Relations

In[]:=