# Ticket #553(closed enhancement: fixed)

Opened 6 years ago

## [with patch, positive review] calling of symbolic expressions is sometimes ridiculous

Reported by: Owned by: was mhansen major sage-2.9 calculus

### Description

The input should be

f = x^(1/9) + (2^(8/9) - 2^(1/9))*(x - 1) - x^(8/9)

Note that * before (x-1).  That your input was accepted is an indication
that SAGE should be more restrictive with what it allows.  What's
happening is that (2^(8/9) - 2^(1/9)) is parsed as a symbolic expression (a
constant function), and then 2^(8/9) - 2^(1/9))(x - 1) is the value of that
constant function at x-1.  Yep, that this is allowed is ridiculous, and should
be changed (I've filed a bug report).

## Change History

### comment:1 Changed 6 years ago by mhansen

• Owner changed from was to mhansen

I attached a patch which throws an error when trying to substitute into a constant expression. It can be overridden by passing a substitute=True parameter to call.

All doc tests pass with it.

### comment:2 Changed 6 years ago by was

• Status changed from new to closed
• Resolution set to fixed

### comment:3 Changed 6 years ago by was

• Status changed from closed to reopened
• Resolution fixed deleted

This patch doesn't fix the problem at all. If you apply it the above example doesn't raise an error. In fact, it's a patch against named substitutions, which should *always* be allowed. So this was totally wrong.

I'm worried actually that the expense of determining whether or not an expression is too costly. The only reasonable fix is to completely ban calling "non-callable" symbolic expressions without an explicit substitution. doing this is a lot more work though.

### comment:4 Changed 6 years ago by was

• Type changed from defect to enhancement
• Milestone changed from sage-2.9 to sage-3

### comment:5 Changed 6 years ago by mabshoff

• Summary changed from calling of symbolic expressions is sometimes ridiculous to [with patch] calling of symbolic expressions is sometimes ridiculous
• Milestone changed from sage-feature to sage-2.8.13

### comment:6 Changed 6 years ago by mabshoff

• Summary changed from [with patch] calling of symbolic expressions is sometimes ridiculous to [with broken patch] calling of symbolic expressions is sometimes ridiculous

### comment:7 Changed 5 years ago by mhansen

I think the best way to go about doing this is to add a number_of_arguments() to SymbolicExpressions?:

sage: sin.number_of_arguments()
1
sage: (sin+1).number_of_arguments()
1
sage: (sin+x).number_of_arguments()
1
sage: (sin+x+y).number_of_arguments()
2
sage: (2^(8/9)-2^(1/9)).number_of_arguments()
0

### comment:8 Changed 5 years ago by mhansen

• Summary changed from [with broken patch] calling of symbolic expressions is sometimes ridiculous to [with patch] calling of symbolic expressions is sometimes ridiculous

I've put up a new patch.

### Changed 5 years ago by was

apply this patch and the one right above it.

### comment:9 Changed 5 years ago by was

• Summary changed from [with patch] calling of symbolic expressions is sometimes ridiculous to [with patch, positive review] calling of symbolic expressions is sometimes ridiculous

### comment:10 Changed 5 years ago by mabshoff

• Status changed from reopened to closed
• Resolution set to fixed

Merged in 2.9.rc0.

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