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Re: Matching with a "Flat" function

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  • Subject: [mg70892] Re: Matching with a "Flat" function
  • From: dh <dh at>
  • Date: Wed, 1 Nov 2006 03:54:58 -0500 (EST)
  • References: <ei4knq$da9$>

if you double the Blank in your pattern it will match the expression:
MatchQ[f[2], f[__Integer]]
Why this is so, I can not tell, it looks like an argument of a function 
with the attribute "Flat" counts double.
This looks like a bug to me.

Szabolcs wrote:
> Could someone explain this behaviour to me? I'd expect f[_Integer] to
> match f[2] even after I set attribute Flat on f.
> Szabolcs
> In[1]:= MatchQ[f[2],f[_]]
> Out[1]= True
> In[2]:= MatchQ[f[2],f[_Integer]]
> Out[2]= True
> In[3]:= SetAttributes[f,Flat]
> In[4]:= MatchQ[f[2],f[_]]
> Out[4]= True
> In[5]:= MatchQ[f[2],f[_Integer]]
> Out[5]= False

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