Wont let me use void F

[benOliveau]

When I try to create a void function called "F" it gives me the following error.

Arduino: 1.5.7 (Windows 8), Board: "Arduino Uno"

In file included from C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/Arduino.h:220:0,
from Morse_Code_Alphabet__input_letters_in_loop_.ino:1:
C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/WString.h:38:28: error: expected unqualified-id before 'reinterpret_cast'
#define F(string_literal) (reinterpret_cast<const _FlashStringHelper *>(PSTR(string_literal)))
^
Morse_Code_Alphabet__input_letters_in_loop.ino:12:6: note: in expansion of macro 'F'
C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/WString.h:38:28: error: expected ')' before 'reinterpret_cast'
#define F(string_literal) (reinterpret_cast<const FlashStringHelper *>(PSTR(string_literal)))
^
Morse_Code_Alphabet__input_letters_in_loop.ino:12:6: note: in expansion of macro 'F'
In file included from C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/Arduino.h:28:0,
from Morse_Code_Alphabet__input_letters_in_loop.ino:1:
C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/WString.h:38:74: error: expected unqualified-id before ')' token
#define F(string_literal) (reinterpret_cast<const FlashStringHelper *>(PSTR(string_literal)))
^
Morse_Code_Alphabet__input_letters_in_loop.ino:12:6: note: in expansion of macro 'F'
In file included from C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/Arduino.h:220:0,
from Morse_Code_Alphabet__input_letters_in_loop.ino:1:
C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/WString.h:38:28: error: expected unqualified-id before 'reinterpret_cast'
#define F(string_literal) (reinterpret_cast<const _FlashStringHelper *>(PSTR(string_literal)))
^
Morse_Code_Alphabet__input_letters_in_loop.ino:79:6: note: in expansion of macro 'F'
C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/WString.h:38:28: error: expected ')' before 'reinterpret_cast'
#define F(string_literal) (reinterpret_cast<const FlashStringHelper *>(PSTR(string_literal)))
^
Morse_Code_Alphabet__input_letters_in_loop.ino:79:6: note: in expansion of macro 'F'
In file included from C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/Arduino.h:28:0,
from Morse_Code_Alphabet__input_letters_in_loop.ino:1:
C:\Program Files (x86)\Arduino\hardware\arduino\avr\cores\arduino/WString.h:38:74: error: expected unqualified-id before ')' token
#define F(string_literal) (reinterpret_cast<const _FlashStringHelper *>(PSTR(string_literal)))
^
Morse_Code_Alphabet__input_letters_in_loop.ino:79:6: note: in expansion of macro 'F'

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[matthijskooijman]

F is a macro, used to mark strings to store in PROGMEM (flash) (e.g. Serial.println(F("Some long string"));). This means you cannot use the identifier F in any other places, this is just an unavoidable limitation. Improving the error message is also tricky, since the compiler doesn't really know what you intended to write.

I guess the best thing we could do is to document this. Perhaps you can point out a place where you would have expected documentation about this?