Expression Handling in JMP: Tips and Trapdoors

[#1]

There is a wonderful lecture by @joseph_morgan  in the Discovery Summit Series:

Using JSL to Develop Efficient, Robust Applications (EU 2018 415) 

Joseph explains the secrets and possibilities of Expression Handling in JSL - and the trapdoors associated with the functionality.

The lecture is based on the content of Expression Handling Functions: Part I - Unraveling the Expr(), NameExpr(), Eval(), ... Conundrum 

further info can be found in the Scripting Guide: advanced-expressions-macros-and-lists 

Nice Course with many application cases: Session 9: Advanced JSL , @jthi 

Basics
internal: #062 - expression handling in JSL 

1	don't evaluate the expression (2+1), just store it in the variable x	x= Expr(2+1)
2	get the expression which is stored in x	Name Expr(x)
3	force evaluation	Eval(myexpr)
4	evaluate xyz and replace Expr(xyz) with the return value	Eval Expr( ... Expr(xyz))

*) NB: in 1 and 4, it's the same function "Expr(...)" - but with the opposite effect:
In 1, the content of Expr doesn't get evaluated, in 4 JUST the content of Expr gets evaluated

3 Universal Tricks

1	pre-evaluate parts of an expression via Eval Expr() ... then evaluate the expression via Eval() solves 90%, (with option 2: 99%) of all issues	Eval(Eval Expr ( ... Exp(eval 1st) ...)) option 2: Eval(Eval Expr ( ... Exp(Name Expr(  )) ...))
2	Solves the same 99% like # 1 - option 2	Eval(substitute(Expr(), Expr(), Name Expr() ))
3	ultimate trick for column references ( @jthi, e.g. Session 9: Advanced JSL @33min )	Name Expr(as column("column name" or :column))

NB:
Some functions evaluate their arguments, some don't, some evaluate some of their arguments - there are functions like head() which evaluate their arguments to a certain degree: Does Head evaluate its argument? - and others evaluate their argument but if it's a single variable, they behave like Name Expr(): Exceptions are the utilities Show, Write, and Print ... 

Unfortunately, most of these details are not available for the users. The users have to guess.

Here is a wish for JSL editor to help the users by showing this information interactively:  Advanced syntax highlighting in JSL Editor - does the function evaluate it's argument? 

-> If you want to get this automatic support, please follow the link and vote!

Examples from the community:

Topic	Links	Details
Column() vs. As column() vs. name()	Why do these ways of referencing columns give different results?How to replace :column_name with variable in JMP JSL?Column() vs As Column() vs datable:column vs dt:As name("column")	the universal syntax to reference a column is :column. But often, it's tricky to get the   ":column" right ...
Column references in column formulas	Insert one expression into another using Eval Insert, Eval Expr, Parse, and Substitute Trying to create a column and fill it with a formula but I get an error. Use expr() to replace variable column names in a formulaAdding Formula Columns based on Column Name ListCreating a formula column from a list ( JSL )Need help with expressions, column references, formulas and column renamingCreate new columns by loop formula calculation with specific column namesJSL to create formula column with variable column names Column name from string  Script concatenate columns  Using Expr() and Eval() with variables name inside a loopUse column variable in IF expression JSL: Creating formula using a variable column name Using variables to create formula columnscreating formula using existing column names  Iterating Columns	define the column via  col = Name Expr(As Column(colname)) and insert it with Eval(Substitute()) Alternatives: As Column (Expr(colname)) with Eval(Eval Expr()) or Expr(Name Expr(AsColumn(dt, colname))) with Eval(Eval Expr()) ...
Column references in platform calls and messages	JSL help proper referencing of a columnRun a Platform without Knowing the Number or Names of Columns Distribution using column list  JSL: help properly referencing a data table column for Distribution platform	use Name Expr(As Column(colname)) with Eval(substitute()) ... or if you have issues with a column list,  just add eval() or  evallist()in the column() argument
use a column reference in a function call	Column reference in custom functionCustom Function - how to reference the column	- wrap the column with  Name Expr() - use a list
construct an expression	Can you construct this without writing expression as a string?Run a Platform without Knowing the Number or Names of ColumnsUsing list of columns in formulas How to script an aggregate column from a variable list of column references	- start with an empty draft and fill it via insert into(draft, arguments) - use substitute - create a list, and substitute list with another JSL symbol
further workarounds via Expression handling	Exclude rows in Col Quantile Calculation	prevent inappropriate caching in Col Quantile by using an expression.
From version to version, less expression handling is needed to make things work	link?

[#2]

Hi @hogi ,

  Thanks for consolidating this information and bringing a lot of this information together. The talk sounds really informative.

  I do like how you can do things several different ways in JMP and in JSL, but sometimes there are too many different ways that are very specific to the case at hand, and this can become somewhat cumbersome when trying to make code that is generic and able to handle many different scenarios.

Thanks!,

DS

[#3]

If you don't use the tricks:

Oops - I lost my data table ...

[#4]

JSL and variables - 3 issue you should be aware off when writing JSL code.

Different issues, always the same solution : )

the solution?

-> just read the main post.

c= 5;
col = Name Expr(:height);
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
gb = graph Builder(); framebox = report(gb)[FrameBox(1)];

 // issues with variables in a message
// all  3 of them look quite innocent 
// but none of them will survive a restart 

 New Column ("rescaled", Formula (c * :height)); 
 dt << New Column ("rescaled", Formula (5 * col));
 framebox << Add Graphics Script(Y Function( c * x, x ) );
 
 // no issue here:
 dt << concatenate(dt);
 dt << new column(c, formula(3));

One could think: don't ever use a variable in a message!
But this goes way too far - the last 2 examples don't have an issue.